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/permissions.test.ts

@@ -3,6 +3,7 @@ import * as core from "@actions/core";
 import { checkWritePermissions } from "../src/github/validation/permissions";
 import type { ParsedGitHubContext } from "../src/github/context";
 import { CLAUDE_APP_BOT_ID, CLAUDE_BOT_LOGIN } from "../src/github/constants";
+import { _resetPlatformCache } from "../src/platform/detector";
 
 describe("checkWritePermissions", () => {
   let coreInfoSpy: any;
@@ -310,6 +311,7 @@ describe("checkWritePermissions", () => {
     let savedServerUrl: string | undefined;
 
     beforeEach(() => {
+      _resetPlatformCache();
       savedPlatform = process.env.PLATFORM;
       savedForgejo = process.env.FORGEJO;
       savedServerUrl = process.env.GITHUB_SERVER_URL;
@@ -324,6 +326,7 @@ describe("checkWritePermissions", () => {
       if (savedServerUrl !== undefined)
         process.env.GITHUB_SERVER_URL = savedServerUrl;
       else delete process.env.GITHUB_SERVER_URL;
+      _resetPlatformCache();
     });
 
     const createForgejoOctokit = (permissions: {