feat(ui): LaunchCmuxButton + useRigCmuxLaunch hook + rig-scope tab-bar trailing slot (slice 24 Checkpoint D)

UI surface for the "Launch in CMUX" feature, completing slice 24's
4-checkpoint chain.

NEW packages/ui/src/hooks/useRigCmuxLaunch.ts:
- useMutation wrapper around POST /api/rigs/:rigId/cmux/launch
- Returns workspaces: CmuxLaunchedWorkspace[] on success
- Throws Error carrying the daemon's honest 3-part message on 4xx/5xx
  (falls back to "HTTP <status>" when body is unparseable)
- Distinct from existing useCmuxLaunch.ts which targets the per-NODE
  focus endpoint POST /api/rigs/:rigId/nodes/:logicalId/focus

NEW packages/ui/src/components/topology/LaunchCmuxButton.tsx:
- Renders button labeled "Launch in CMUX" (per README §Button label
  DECIDED); "Launching…" while in-flight; disabled during pending
- Inline status surface below the button: success message names the
  workspace(s) + agent count; error message surfaces the daemon's
  honest 3-part error verbatim (status persists for 6s success /
  8s error, then auto-clears via setTimeout)
- Mobile-hide via Tailwind `hidden lg:inline-flex` per README §Mobile

MODIFY packages/ui/src/components/topology/TopologyViewModeTabs.tsx:
- New optional `trailing?: ReactNode` prop rendered with ml-auto inside
  the tablist flex container. Allows scope pages to mount tab-bar-far-
  right elements (e.g., the slice 24 Launch button per README §Button
  placement Option C). Other scope pages that don't pass trailing
  render unchanged.
- Existing test-id contracts preserved; new `${testIdPrefix}-trailing`
  data-testid on the slot wrapper.

MODIFY packages/ui/src/components/topology/ScopePages.tsx:
- RigScopePage's <TopologyViewModeTabs> now passes trailing={<LaunchCmuxButton rigId={rigId} />}
- HostScopePage + PodScopePage + SeatScopePage unchanged (no trailing
  slot consumed); button is rig-scope-only per README §Button placement.

Tests added:
- 4/4 useRigCmuxLaunch hook tests (POST shape, success workspaces[],
  error path, network failure path)
- 7/7 LaunchCmuxButton tests (label, mobile-hide class, click POSTs,
  loading state, success toast with workspace + agent count, honest
  3-part error toast, error contains actionable hint)

Total UI tests: 765 pass + 1 skipped + 52 pre-existing fails on main
HEAD baseline (verified via stash-and-recheck: failure count identical
with and without this commit's UI changes; AppShell global-polls
leakage per banked feedback_appshell_global_polls_test_pollution).
Net: +11 passing tests; zero new failures.

Typecheck: all 4 touched UI files clean.

This completes slice 24's UI scope:
- Checkpoint A: CmuxAdapter RPC method extensions + transport mapping
- Checkpoint B: CmuxLayoutService (algorithm + chunking + ordering +
  buildWorkspace with timing gotchas)
- Checkpoint C: POST /api/rigs/:rigId/cmux/launch + NodeInventory wiring
- Checkpoint D: UI button + hook + tab-bar trailing slot (this commit)

Next: slice-close ACK + walk packets + final hand-off DM to orch-lead
+ velocity-qa dispatch for Tart VM clean-install proof (HG-11/12/13
remain founder-host QA per README §QA-lane).

SC-29 EXCEPTION declared: NONE at this checkpoint
(UI consumes the SC-29 #10 daemon endpoint declared at Checkpoint C
a18e997; no new vector added.)

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

packages/ui/src/components/topology/LaunchCmuxButton.tsx

@@ -0,0 +1,95 @@
+// Slice 24 — LaunchCmuxButton.
+//
+// Rig-scope "Launch in CMUX" button. Click → POST /api/rigs/:rigId/cmux/launch
+// via useRigCmuxLaunch. Renders inline status (loading / success / error)
+// directly adjacent to the button rather than via a separate toast
+// primitive — keeps the rig-scope tab-bar self-contained.
+//
+// Per README §Mobile + §Button placement: mounted in the rig-scope tab
+// bar (Option C placement, persistent across all rig-scope view-mode
+// tabs); hidden below the lg breakpoint via Tailwind responsive classes.
+
+import { useEffect, useState } from "react";
+import { useRigCmuxLaunch } from "../../hooks/useRigCmuxLaunch.js";
+
+interface LaunchCmuxButtonProps {
+  rigId: string;
+}
+
+type StatusKind = "idle" | "loading" | "success" | "error";
+
+interface StatusState {
+  kind: StatusKind;
+  message?: string;
+}
+
+const SUCCESS_TOAST_TIMEOUT_MS = 6000;
+const ERROR_TOAST_TIMEOUT_MS = 8000;
+
+export function LaunchCmuxButton({ rigId }: LaunchCmuxButtonProps) {
+  const mutation = useRigCmuxLaunch();
+  const [status, setStatus] = useState<StatusState>({ kind: "idle" });
+
+  useEffect(() => {
+    if (status.kind === "idle" || status.kind === "loading") return;
+    const timeout = status.kind === "success" ? SUCCESS_TOAST_TIMEOUT_MS : ERROR_TOAST_TIMEOUT_MS;
+    const t = window.setTimeout(() => setStatus({ kind: "idle" }), timeout);
+    return () => window.clearTimeout(t);
+  }, [status]);
+
+  const handleClick = () => {
+    if (mutation.isPending) return;
+    setStatus({ kind: "loading" });
+    mutation
+      .mutateAsync({ rigId })
+      .then((result) => {
+        const workspaceCount = result.workspaces.length;
+        const agentCount = result.workspaces.reduce((sum, w) => sum + w.agents.length, 0);
+        const names = result.workspaces.map((w) => w.name).join(", ");
+        setStatus({
+          kind: "success",
+          message:
+            workspaceCount === 1
+              ? `Launched cmux workspace "${names}" with ${agentCount} agent${agentCount === 1 ? "" : "s"}.`
+              : `Launched ${workspaceCount} cmux workspaces (${names}) with ${agentCount} agents total.`,
+        });
+      })
+      .catch((err: Error) => {
+        setStatus({ kind: "error", message: err.message });
+      });
+  };
+
+  const buttonLabel = mutation.isPending ? "Launching…" : "Launch in CMUX";
+
+  return (
+    <div
+      data-testid="launch-cmux-wrapper"
+      className="hidden lg:inline-flex items-center gap-3 ml-auto"
+    >
+      <button
+        type="button"
+        data-testid="launch-cmux-button"
+        onClick={handleClick}
+        disabled={mutation.isPending}
+        className="border border-stone-700 bg-white px-3 py-2 font-mono text-[10px] uppercase tracking-[0.18em] text-stone-900 hover:bg-stone-100 disabled:opacity-50 disabled:cursor-not-allowed focus:outline-none focus:ring-1 focus:ring-stone-400"
+      >
+        {buttonLabel}
+      </button>
+      {status.kind !== "idle" && status.message ? (
+        <span
+          data-testid="launch-cmux-status"
+          data-status-kind={status.kind}
+          role="status"
+          aria-live="polite"
+          className={
+            status.kind === "error"
+              ? "font-mono text-[10px] text-rose-700 max-w-md truncate"
+              : "font-mono text-[10px] text-emerald-700 max-w-md truncate"
+          }
+        >
+          {status.message}
+        </span>
+      ) : null}
+    </div>
+  );
+}

packages/ui/src/components/topology/ScopePages.tsx

@@ -17,6 +17,7 @@ import {
   type TopologySeatScopeTab,
 } from "./TopologyViewModeTabs.js";
 import { TopologyTableView } from "./TopologyTableView.js";
+import { LaunchCmuxButton } from "./LaunchCmuxButton.js";
 import { TopologyTerminalView } from "./TopologyTerminalView.js";
 import { SectionHeader } from "../ui/section-header.js";
 import { EmptyState } from "../ui/empty-state.js";
@@ -148,7 +149,15 @@ export function RigScopePage() {
     <ScopeShell
       eyebrow="Topology · Rig"
       title={rig?.name ?? rigId}
-      tabsNav={<TopologyViewModeTabs tabs={RIG_POD_SCOPE_TABS} active={active} onSelect={setActive} testIdPrefix="topology-rig" />}
+      tabsNav={
+        <TopologyViewModeTabs
+          tabs={RIG_POD_SCOPE_TABS}
+          active={active}
+          onSelect={setActive}
+          testIdPrefix="topology-rig"
+          trailing={<LaunchCmuxButton rigId={rigId} />}
+        />
+      }
     >
       {effectiveActive === "graph" ? (
         <div className="flex-1 min-h-0 relative">

packages/ui/src/components/topology/TopologyViewModeTabs.tsx

@@ -7,6 +7,7 @@
 // State managed via React (useState in scope page); URL stays at the
 // scope path (/topology, /topology/rig/$rigId, etc).
 
+import type { ReactNode } from "react";
 import { cn } from "../../lib/utils.js";
 
 export type TopologyHostScopeTab = "graph" | "table" | "terminal";
@@ -22,6 +23,13 @@ interface TopologyViewModeTabsProps<T extends string> {
   active: T;
   onSelect: (id: T) => void;
   testIdPrefix?: string;
+  /**
+   * Slice 24 — optional trailing slot rendered with ml-auto inside the
+   * tab-bar flex container. Used by RigScopePage to render the
+   * "Launch in CMUX" button at the tab-bar far right per README §Button
+   * placement Option C (persistent across all rig-scope view-mode tabs).
+   */
+  trailing?: ReactNode;
 }
 
 export function TopologyViewModeTabs<T extends string>({
@@ -29,6 +37,7 @@ export function TopologyViewModeTabs<T extends string>({
   active,
   onSelect,
   testIdPrefix = "topology-view-mode",
+  trailing,
 }: TopologyViewModeTabsProps<T>) {
   return (
     // Internal tablist — div, not <nav>, so SC-1 left-chrome count
@@ -40,7 +49,7 @@ export function TopologyViewModeTabs<T extends string>({
       role="tablist"
       aria-label="Topology view modes"
       data-testid={`${testIdPrefix}-tabs`}
-      className="flex gap-6"
+      className="flex gap-6 items-center"
     >
       {tabs.map((t) => (
         <button
@@ -61,6 +70,11 @@ export function TopologyViewModeTabs<T extends string>({
           {t.label}
         </button>
       ))}
+      {trailing ? (
+        <div data-testid={`${testIdPrefix}-trailing`} className="ml-auto">
+          {trailing}
+        </div>
+      ) : null}
     </div>
   );
 }

packages/ui/src/hooks/useRigCmuxLaunch.ts

@@ -0,0 +1,55 @@
+// Slice 24 — useRigCmuxLaunch.
+//
+// Mutation hook that POSTs to /api/rigs/:rigId/cmux/launch (the new
+// daemon endpoint shipped in slice 24 Checkpoint C). Returns the
+// workspaces array from the daemon's response on success; throws an
+// Error carrying the daemon's honest 3-part message on 4xx/5xx.
+//
+// Distinct from the existing useCmuxLaunch.ts which targets the
+// per-NODE focus endpoint POST /api/rigs/:rigId/nodes/:logicalId/focus.
+
+import { useMutation } from "@tanstack/react-query";
+
+export interface RigCmuxLaunchInput {
+  rigId: string;
+}
+
+export interface CmuxLaunchedWorkspace {
+  name: string;
+  agents: string[];
+  blanks: number;
+}
+
+export interface RigCmuxLaunchSuccess {
+  ok: true;
+  workspaces: CmuxLaunchedWorkspace[];
+}
+
+interface RigCmuxLaunchErrorBody {
+  error: string;
+  message: string;
+  partial?: CmuxLaunchedWorkspace[];
+}
+
+async function postRigCmuxLaunch({ rigId }: RigCmuxLaunchInput): Promise<RigCmuxLaunchSuccess> {
+  const res = await fetch(`/api/rigs/${encodeURIComponent(rigId)}/cmux/launch`, {
+    method: "POST",
+  });
+  if (!res.ok) {
+    let body: RigCmuxLaunchErrorBody | null = null;
+    try {
+      body = (await res.json()) as RigCmuxLaunchErrorBody;
+    } catch {
+      // fall through
+    }
+    const message = body?.message ?? body?.error ?? `HTTP ${res.status}`;
+    throw new Error(message);
+  }
+  return (await res.json()) as RigCmuxLaunchSuccess;
+}
+
+export function useRigCmuxLaunch() {
+  return useMutation<RigCmuxLaunchSuccess, Error, RigCmuxLaunchInput>({
+    mutationFn: postRigCmuxLaunch,
+  });
+}