feat(prd-1): implement M5 - Forgejo comment URL formatting

- Create src/forgejo/comment-logic.ts with Forgejo-specific URL builders:
  - Unicode spinner (replaces GitHub CDN image)
  - PR compare URL with expand=1 (no quick_pull=1)
  - Dynamic extractOwnerRepoFromUrl() fixing hardcoded github.com regex
  - updateForgejoCommentBody() with cross-platform URL extraction
- 41 tests covering all URL patterns, spinner, and the critical regex fix
- No upstream files modified (lean adapter approach)

Progress: 56% complete (5/9) - next: M6 (MCP server platform routing)

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

prds/1-forgejo-platform-support.md

@@ -263,7 +263,7 @@ The upstream repo has 13 workflows in `.github/workflows/`. Strategy:
   - Depends on: M4a
   - Success: Full parity with GitHub GraphQL data fetch. All comments, reviews, and inline comments included with proper TOCTOU filtering.
 
-- [ ] **M5: Forgejo comment URL formatting** — Create `src/forgejo/comment-logic.ts` with Forgejo-specific URL patterns for job links, branch links, and PR creation links (used in tracking comment updates).
+- [x] **M5: Forgejo comment URL formatting** — Create `src/forgejo/comment-logic.ts` with Forgejo-specific URL patterns for job links, branch links, and PR creation links (used in tracking comment updates).
   - Files: `src/forgejo/comment-logic.ts` (NEW)
   - Depends on: M1
   - Success: Comment formatting produces correct Forgejo URLs.