test(tui): cover the /undo numeric-sequence form (#698 review)

Add a parseCommand("/undo 123") assertion so both the latest and numeric
<sequence> forms of the /rewind contract are exercised through the alias.

internal/tui/commands.go

@@ -253,6 +253,7 @@ var commandDefinitions = []commandDefinition{
 	},
 	{
 		name:        "/rewind",
+		aliases:     []string{"/undo"},
 		usage:       "/rewind [latest|<sequence>]",
 		group:       commandGroupSession,
 		description: "Restore workspace files to a checkpoint and truncate the session.",

internal/tui/commands_test.go

@@ -985,6 +985,31 @@ func TestParseBackgroundTerminalCommands(t *testing.T) {
 	}
 }
 
+// /undo is an alias for /rewind (issue #698): previously it fell through to
+// commandUnknown.
+func TestUndoAliasesRewind(t *testing.T) {
+	rewind, ok := resolveCommand("/rewind")
+	if !ok {
+		t.Fatal("expected /rewind to resolve")
+	}
+	alias, ok := resolveCommand("/undo")
+	if !ok || alias.kind != rewind.kind {
+		t.Fatalf("expected /undo to alias /rewind, got ok=%v command=%#v", ok, alias)
+	}
+	// End-to-end: a typed /undo dispatches to the rewind handler with args intact,
+	// for both the "latest" and numeric-<sequence> forms of the /rewind contract.
+	if parsed := parseCommand("/undo latest"); parsed.kind != commandRewind || parsed.text != "latest" {
+		t.Fatalf("parseCommand(%q) = %#v, want commandRewind with text=latest", "/undo latest", parsed)
+	}
+	if parsed := parseCommand("/undo 123"); parsed.kind != commandRewind || parsed.text != "123" {
+		t.Fatalf("parseCommand(%q) = %#v, want commandRewind with text=123", "/undo 123", parsed)
+	}
+	// Guard: without the alias it would be commandUnknown.
+	if parsed := parseCommand("/undo"); parsed.kind == commandUnknown {
+		t.Fatal("/undo resolved to commandUnknown — the alias is not wired")
+	}
+}
+
 func TestCommandSelectionRequiresInputFromUsage(t *testing.T) {
 	cases := []struct {
 		name string