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>

docs/forgejo.md

@@ -88,6 +88,8 @@ The action detects the platform using a three-tier strategy:
 
 **Recommendation**: Always set `platform: "forgejo"` explicitly. Auto-detection works but the explicit input eliminates ambiguity.
 
+> **Note for GitHub Enterprise Server (GHES) users**: The tier-3 heuristic classifies any non-`github.com` URL as Forgejo. If you run on GHES (e.g., `https://github.mycompany.com`), you **must** set `platform: "github"` explicitly to avoid misdetection.
+
 ## Differences from GitHub
 
 ### Authentication
@@ -340,6 +342,7 @@ All execution modes and trigger types have been validated end-to-end on a Forgej
 
 ## Known Limitations
 
+- **GitHub Enterprise Server (GHES) misdetection**: The tier-3 platform detection heuristic treats any `GITHUB_SERVER_URL` that isn't `github.com` as Forgejo. GHES instances (e.g., `https://github.mycompany.com`) will be misidentified. Set `platform: "github"` explicitly when running on GHES.
 - **Git ref API**: Forgejo uses `/git/refs` (plural) instead of GitHub's `/git/ref` (singular). The action handles this by using `repos.getBranch()` on Forgejo for branch SHA lookups.
 - **User display name fetch**: `GET /users/{username}` returns 403 without `read:user` scope. Non-fatal — falls back to login name.
 - **`pull_request_target` event**: May not exist in Forgejo. The action degrades gracefully (maps to `pull_request`).