Let the operator cancel the most recently queued message

[TheGreatAxios]

Summary

• Ctrl+G cancels the newest queued or steered message: retracts it from SessionQueueState and marks its transcript row with a cancelled flag. paintStreamRow reads that flag and renders the [cancelled] prefix for user rows — row.text itself stays exactly what the operator typed, so copy-mode and anything else reading text sees the original message, not a mutated one.
• Keybinding: Ctrl+G. Unclaimed by both TextareaRenderable's default bindings (checked against the bundled defaultTextareaKeyBindings table) and this shell's other chords. Readline/Emacs users already read it as "abort" — unlike Ctrl+X, the first attempt's pick, which is cut/prefix muscle memory.
• Scope: last-only. cancelLast retracts the newest item regardless of kind (queue or steer). Reaching an earlier item would need a picker overlay — not justified without an operator asking for it. Editing a queued message (also named in CL-5563) stays out of scope.

Verification

• bun run test — 3970 pass, 0 fail
• bun run typecheck — clean
• bun run build — clean
• Regression tests assert on the rendered frame (captureCharFrame()), not only shell.streamLog — the exact gap that let the first attempt through. Covered for both queue- and steer-kind cancels, in session-queue.test.ts, shell.test.ts, and keybindings.test.ts.
• Rendered before/after, withTestRenderer:

Before:

▍ keep this one
▍ oops wrong message
 queue 2

Row model: [{"text":"keep this one","meta":"queue"},{"text":"oops wrong message","meta":"queue"}]

After applyShellCancelLast:

▍ keep this one
▍ [cancelled] oops wrong message
 queue 1

Row model: [{"text":"keep this one","meta":"queue","cancelled":undefined},{"text":"oops wrong message","meta":"cancelled","cancelled":true}] — text is unchanged; only the flag and the paint output differ.

Closes CL-5572

[linear-code]

CL-5572