fix(prd-7): M1 architecture review — consolidate duplication, fix bugs, memoize detection

- F1: Consolidated updateForgejoCommentBody into unified updateCommentBody (~150 lines removed)
- F2: Fixed wrong run identifier in update-with-branch.ts (context.runId → getForgejoRunIndex())
- F3: Memoized detectPlatform() with _resetPlatformCache() for tests
- F4: Added getServerUrl() runtime helper to resolve constant-vs-function inconsistency
- F5: Fixed sticky comment bot matching on Forgejo via context.inputs.botName
- F6: Documented GHES heuristic misidentification in docs/forgejo.md
- Updated PRD M1 checkbox to complete

Progress: 17% complete — M1 done, M2 (error handling review) next

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

prds/7-senior-review-forgejo.md

@@ -58,7 +58,7 @@ Perform a thorough, documented code review of all Forgejo-related changes. The r
 
 ## Milestones
 
-- [ ] **M1: Architecture & design review**
+- [x] **M1: Architecture & design review**
 
   - Review the lean adapter pattern: is it the right abstraction level?
   - Assess merge-conflict surface with upstream: are platform branches minimal?