fix(prd-7): M4 test coverage review — 28 new tests for comment operations and branch setup

- test/create-initial-comment.test.ts: 15 tests covering GitHub/Forgejo
  platform branches, sticky comment matching, review comment fallback,
  error handling with GITHUB_OUTPUT verification
- test/update-tracking-comment.test.ts: 11 tests covering body construction,
  branch link logic (issue vs PR), comment type routing, Forgejo URLs
- test/setup-branch-forgejo.test.ts: 2 tests verifying Forgejo uses
  repos.getBranch() while GitHub uses git.getRef() for source branch SHA
- test/COVERAGE-NOTES.md: documents all coverage status and accepted gaps
  (tag mode orchestrator, entrypoint) with rationale
- Notable finding: CLAUDE_APP_BOT_ID discrepancy flagged for M5

Progress: 67% complete (4/6 milestones) — M5 code quality next

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

prds/7-senior-review-forgejo.md

@@ -84,7 +84,7 @@ Perform a thorough, documented code review of all Forgejo-related changes. The r
   - Secrets in logs: review all `core.info()` / `core.debug()` calls
   - **Deliverable**: Security findings with severity ratings
 
-- [ ] **M4: Test coverage review & improvements**
+- [x] **M4: Test coverage review & improvements**
 
   - Assess existing unit test coverage for Forgejo code paths
   - Identify critical paths without test coverage