feat: resolve discussions instead of delete (opt-in cleanup_mode)

[brucearctor]

Context

Currently CleanPreviousReviews deletes all old bot comments. This destroys review history.

GitLab supports resolving discussion threads (PUT /discussions/:id { resolved: true }), which collapses them instead of removing them. Users can still see previous review feedback.

Proposed Changes

1. Add cleanup_mode config field: resolve | delete (default: delete for backward compat)
2. GitLab: resolve bot discussions via PUT /discussions/:id
3. GitHub: keep delete behavior (no thread resolution via REST API)
4. Delete bot summary notes (non-discussion notes can't be resolved)

E9 Note

Resolve-instead-of-delete is a silent behavior change. Must be opt-in, not a default flip.

Ref

E9 design session in conversation 8ecd8c13