feat(lastcode): integrate resumable actions with composer stack

[lastobelus]

Resumable Actions still used the deleted legacy composer attachment model, while annotations and message-sync state competed for the same space outside the restored upstream composer stack. Completed Actions also flattened their structured findings into plain text.

This moves live Actions, thread annotations, and message-sync status onto the shared composer banner stack. Expanded live items detach above the composer; completed Action results remain collapsed, outcome-colored, expandable cards in the thread on web and mobile. The stack now expands through pointer or focus interaction and wraps multi-action controls below content on narrow screens. The old Action drawer, annotation post-it, sync pill, and their legacy styling are removed rather than restored.

Validation:

• Quick CI passed for 5728224b8d0e3a6f23d681ccf454e994b04972da against 398a2ede369486a9f791019910228e29557ff749
• 123 focused tests passed across Action Resume, composer banners, annotations, timeline rendering, completion details, and mobile external links
• shared, web, and mobile typechecks passed
• focused formatting and lint passed (apart from one unchanged mobile unused-variable warning)
• integrated desktop and 390px visual QA passed; screenshots are attached in the PR thread

Implemented by GPT-5.6-sol in the T3 Code Codex harness.