fix(prd-7): M2 error handling review — fix process.exit bypasses, TOCTOU gap, URL consistency

- F1: Replace process.exit(1) with throw in branch.ts (cleanup was bypassed)
- F2: Replace process.exit(1) with throw in install-mcp-server.ts (same issue)
- F2b: Replace process.exit(1) with throw in create-prompt/index.ts (same issue)
- F3: Use getServerUrl() instead of GITHUB_SERVER_URL constant in branch-cleanup.ts
- F4: Use updated_at for review TOCTOU filtering in Forgejo data fetcher
- Accepted notes: no pagination >100, no retry, unconditional trigger, image no-op

Progress: 33% complete — M2 done, M3 (security review) next

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

prds/7-senior-review-forgejo.md

@@ -67,7 +67,7 @@ Perform a thorough, documented code review of all Forgejo-related changes. The r
   - Evaluate whether the `src/forgejo/` directory structure scales
   - **Deliverable**: Architecture findings document with severity ratings
 
-- [ ] **M2: Error handling & edge case review**
+- [x] **M2: Error handling & edge case review**
 
   - Review every `try/catch` block in Forgejo code paths
   - Check for unhandled promise rejections