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/actor.test.ts

@@ -2,6 +2,7 @@
 
 import { describe, test, expect, beforeEach, afterEach } from "bun:test";
 import { checkHumanActor } from "../src/github/validation/actor";
+import { _resetPlatformCache } from "../src/platform/detector";
 import type { Octokit } from "@octokit/rest";
 import { createMockContext } from "./mockContext";
 
@@ -116,6 +117,7 @@ describe("checkHumanActor", () => {
     let savedServerUrl: string | undefined;
 
     beforeEach(() => {
+      _resetPlatformCache();
       savedPlatform = process.env.PLATFORM;
       savedServerUrl = process.env.GITHUB_SERVER_URL;
       process.env.PLATFORM = "forgejo";
@@ -127,6 +129,7 @@ describe("checkHumanActor", () => {
       if (savedServerUrl !== undefined)
         process.env.GITHUB_SERVER_URL = savedServerUrl;
       else delete process.env.GITHUB_SERVER_URL;
+      _resetPlatformCache();
     });
 
     test("should skip actor check entirely on Forgejo", async () => {