feat: restore work-mode message delete with cache-break confirmation (#1218)

* feat: restore message deletion in work mode

Work-mode conversations keep their other structural restrictions, but
single-message delete (including compacted summaries) is available again
so users can remove unwanted turns without leaving the session.

Co-authored-by: Zeng Xian <themezeng@gmail.com>

* docs: clarify work-mode summary delete comment

Co-authored-by: Zeng Xian <themezeng@gmail.com>

* feat: confirm cache-breaking edits in long work-mode chats

Work-mode conversations that already have a cached prefix now ask before
deleting an earlier message or switching models, so users see the token
and latency cost of invalidating that cache.

Co-authored-by: Zeng Xian <themezeng@gmail.com>

* fix: make prompt cache confirmation context-aware

* fix: recheck cache policy before message deletion

* feat: add don't-show-again to work-mode cache confirmation

Persist a single setting so users can skip later cache-break prompts
after confirming delete, model switch, or summary restore.

Co-authored-by: Zeng Xian <themezeng@gmail.com>

* fix: align cache confirmation with prompt context

* fix: confirm only context-changing deletions

* fix: scope cache evidence to selected context

* docs: describe context-scoped work-mode cache confirmation

Co-authored-by: Zeng Xian <themezeng@gmail.com>

* feat: show work-mode summary edit and split cache-confirm dismissals

Keep latest-summary edit visible in work mode. Remember Don't show again
per scene in uiStore instead of a single settings flag.

Co-authored-by: Zeng Xian <themezeng@gmail.com>

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Co-authored-by: Cursor Agent <cursoragent@cursor.com>

(cherry picked from pro commit 27e15d8efe9744207ca49306c3dfa9814335eae1)