fix(tui): show the interrupted-session hint inside the TUI (Refs #7095)

A plain `codewhale` launch deliberately starts fresh. When it found an
in-flight session snapshot (a session whose turn never finished, which
exists only under ~/.codewhale/sessions/checkpoints/), the only pointer to
`codewhale --continue` was an eprintln! printed just before the TUI took
the alternate screen, so a user who closed the terminal mid-task reopened
to an empty session and no visible way back (#7095).
`preserve_interrupted_checkpoint_for_explicit_resume` now returns that
message and the launch passes it as the TUI startup notice; an auto-resume
receipt still takes precedence. Its doc comment no longer claims
checkpoints are persisted as sessions (untrue since #4479).

Also applies rustfmt to the `mod usage_persistence` line 98104b9737
left out of order, the only `cargo fmt --all -- --check` drift on
origin/main at a19f2d8885.

Not changed here (follow-ups): the SIGHUP/SIGTERM path exits without
flushing the persistence actor; the resume picker and `codewhale sessions`
do not list checkpoint-only sessions; the 24h checkpoint prune can remove
them.

Evidence (on an export of #7108's merged head cee2d8ab7c plus these lib.rs
hunks, since origin/main codewhale-tui did not compile at the time):
- scripts/dev-cargo.sh check -p codewhale-tui --lib --locked: rc=0, no
  errors or warnings.
- scripts/dev-cargo.sh test -p codewhale-tui --lib --locked -- checkpoint:
  test result: ok. 79 passed; 0 failed (includes
  plain_launch_preserves_checkpoint_but_starts_fresh,
  plain_launch_consumes_legacy_checkpoint_after_preserving_it,
  continue_without_interactive_terminal_leaves_checkpoint_for_a_real_launch).
- cargo fmt --all -- --check on origin/main + this commit: clean.
Not run: a live TUI launch with a real interrupted session.

CW-7525.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- **Interrupted session hint stays visible (#7095):** when a plain `codewhale` launch finds an in-flight session snapshot, the "Run `codewhale --continue` to resume it" hint now appears in the TUI's status line instead of being printed to the terminal just before the TUI covered it.
 - **Crate split S24a (internal):** the workshop byte-budget slot moves to `codewhale-runtime`, `AutoRouteDecisionEvidence` to `codewhale-models`, and cost-status session persistence to a tui `usage_persistence` module, so billing no longer reaches the session manager or model routing. No behavior change. (CW-6129)
 - **Nested and deletable threads (Runtime API):** a fork (including a backtrack fork) records `parent_thread_id`, `POST /v1/threads` accepts one, and `GET /v1/threads/summary` returns it so clients can nest child conversations. `DELETE /v1/threads/{id}` removes an idle thread with its turns, items, events and goal, and its saved session unless another thread shares it or a terminal holds it open; a running thread answers 409.
 - **Hunk actions in Review (Runtime API):** `POST /v1/git/stage`, `/v1/git/unstage` and `/v1/git/discard` take `hunks` (the `@@ … @@` headers from `GET /v1/diff`) with one path and apply only those hunks through `git apply`, honouring the same `expect` preconditions as whole-path writes. A hunk that no longer exists answers `409 git_state_changed` with `stale_hunks`, and nothing is written unless git accepts the whole patch.

crates/tui/CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- **Interrupted session hint stays visible (#7095):** when a plain `codewhale` launch finds an in-flight session snapshot, the "Run `codewhale --continue` to resume it" hint now appears in the TUI's status line instead of being printed to the terminal just before the TUI covered it.
 - **Crate split S24a (internal):** the workshop byte-budget slot moves to `codewhale-runtime`, `AutoRouteDecisionEvidence` to `codewhale-models`, and cost-status session persistence to a tui `usage_persistence` module, so billing no longer reaches the session manager or model routing. No behavior change. (CW-6129)
 - **Nested and deletable threads (Runtime API):** a fork (including a backtrack fork) records `parent_thread_id`, `POST /v1/threads` accepts one, and `GET /v1/threads/summary` returns it so clients can nest child conversations. `DELETE /v1/threads/{id}` removes an idle thread with its turns, items, events and goal, and its saved session unless another thread shares it or a terminal holds it open; a running thread answers 409.
 - **Hunk actions in Review (Runtime API):** `POST /v1/git/stage`, `/v1/git/unstage` and `/v1/git/discard` take `hunks` (the `@@ … @@` headers from `GET /v1/diff`) with one path and apply only those hunks through `git apply`, honouring the same `expect` preconditions as whole-path writes. A hunk that no longer exists answers `409 git_state_changed` with `stale_hunks`, and nothing is written unless git accepts the whole patch.

crates/tui/src/lib.rs

@@ -35,7 +35,6 @@ mod conformance;
 mod context_report;
 mod core;
 mod cost_status;
-mod usage_persistence;
 /// Guarded installation I/O reusing Engine's confined file primitives.
 pub mod delivery_files;
 mod doctor;
@@ -46,6 +45,7 @@ mod extension_host;
 mod fleet;
 mod git_status;
 mod keybinding_table;
+mod usage_persistence;
 use crate::fleet::executor::exec_stream_final_answer_excerpt;
 mod hooks;
 mod image_attach;
@@ -3104,14 +3104,14 @@ async fn run_async_main_dispatch(
         Some(id)
     } else if !cli.fresh {
         let workspace = resolve_workspace(&cli);
-        preserve_interrupted_checkpoint_for_explicit_resume(&workspace);
+        let checkpoint_notice = preserve_interrupted_checkpoint_for_explicit_resume(&workspace);
         // Opt-in auto-resume (#2934). Off by default, so the historical
         // "plain `codewhale` starts fresh" behaviour is unchanged unless the
         // user asked for something else. The decision never resumes an
         // archived, unreadable, or foreign-workspace session; every fallback
         // carries a receipt rather than silently starting blank.
         let (session_id, notice) = resolve_auto_resume(&workspace);
-        startup_notice = notice;
+        startup_notice = notice.or(checkpoint_notice);
         session_id
     } else {
         None
@@ -13115,18 +13115,14 @@ fn saved_session_is_newer(
 /// the same folder" from re-entering the previous in-flight session while still
 /// leaving an explicit resume path.
 ///
-/// Only the newest recent checkpoint drives the notice. The legacy
-/// single-slot file is persisted as a regular session and consumed (today's
-/// behavior for that slot); per-session checkpoint files are persisted but
-/// left in place — they may belong to a live session in another terminal,
-/// and `--continue` reads them directly.
-fn preserve_interrupted_checkpoint_for_explicit_resume(launch_workspace: &Path) {
-    let Some(manager) = session_manager::SessionManager::default_location().ok() else {
-        return;
-    };
-    let Some(newest) = load_recent_checkpoints(&manager).into_iter().next() else {
-        return;
-    };
+/// Only the newest recent checkpoint drives the notice, which is returned for
+/// the TUI to show. The legacy single-slot file is migrated to a per-session
+/// checkpoint and consumed; per-session checkpoint files are left in place —
+/// they may belong to a live session in another terminal, and `--continue`
+/// reads them directly.
+fn preserve_interrupted_checkpoint_for_explicit_resume(launch_workspace: &Path) -> Option<String> {
+    let manager = session_manager::SessionManager::default_location().ok()?;
+    let newest = load_recent_checkpoints(&manager).into_iter().next()?;
 
     let session_workspace = newest.session.metadata.workspace.clone();
     // #4479: removed save_session call — checkpoint should not be auto-promoted to session
@@ -13139,20 +13135,22 @@ fn preserve_interrupted_checkpoint_for_explicit_resume(launch_workspace: &Path)
     }
 
     let age_str = checkpoint_age_label(newest.age);
-    if session_manager::workspace_scope_matches(&session_workspace, launch_workspace) {
-        eprintln!(
-            "Found an in-flight session snapshot ({age_str}). Starting a new \
-             session. Run `codewhale --continue` to resume it."
-        );
-    } else {
-        eprintln!(
-            "Note: an interrupted session from another workspace ({}) is \
-             available. Run `codewhale sessions` to list saved sessions. Starting \
-             fresh in {}.",
-            session_workspace.display(),
-            launch_workspace.display(),
-        );
-    }
+    Some(
+        if session_manager::workspace_scope_matches(&session_workspace, launch_workspace) {
+            format!(
+                "Found an in-flight session snapshot ({age_str}). Starting a new \
+                 session. Run `codewhale --continue` to resume it."
+            )
+        } else {
+            format!(
+                "Note: an interrupted session from another workspace ({}) is \
+                 available. Run `codewhale sessions` to list saved sessions. Starting \
+                 fresh in {}.",
+                session_workspace.display(),
+                launch_workspace.display(),
+            )
+        },
+    )
 }
 
 /// Load project-level config from `$WORKSPACE/.codewhale/config.toml`, with