fix(prd-7): M3 security review — self-trigger guard, actor permissions, trigger bypass, platform validation

- Add self-mention/bot loop prevention in actor.ts (blocks actor matching bot_name)
- Switch Forgejo permission check to actor-level via getCollaboratorPermissionLevel
  with fallback to token-level repos.get() when collaborator API returns 403
- Close empty-body PR review comment trigger bypass (now returns false instead of true)
- Add platform detection cross-validation (reject PLATFORM=forgejo on github.com)
- Document URL builder defense-in-depth rationale in comment-logic.ts
- Update/add tests across 6 test files (actor, permissions, trigger, platform, mcp, m7)

Progress: 50% complete (3/6 milestones) — next: M4 test coverage review

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prds/7-senior-review-forgejo.md

@@ -76,7 +76,7 @@ Perform a thorough, documented code review of all Forgejo-related changes. The r
   - Review fallback behavior (e.g., `repos.get()` fallback for permissions)
   - **Deliverable**: List of error handling gaps with fixes
 
-- [ ] **M3: Security review**
+- [x] **M3: Security review**
 
   - Token handling: is the token properly scoped and never logged?
   - Input sanitization: can webhook payloads inject content?