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>

test/platform-detector.test.ts

@@ -1,9 +1,10 @@
-import { detectPlatform } from "../src/platform/detector";
+import { detectPlatform, _resetPlatformCache } from "../src/platform/detector";
 import { describe, it, expect, beforeEach } from "bun:test";
 
 describe("detectPlatform", () => {
   beforeEach(() => {
-    // Reset relevant env vars before each test
+    // Reset memoized result and env vars before each test
+    _resetPlatformCache();
     delete process.env.PLATFORM;
     delete process.env.FORGEJO;
     delete process.env.GITHUB_SERVER_URL;