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>

src/entrypoints/update-comment-link.ts

@@ -8,7 +8,6 @@ import {
   type CommentUpdateInput,
 } from "../github/operations/comment-logic";
 import {
-  updateForgejoCommentBody,
   createForgejoPrCompareUrl,
   getForgejoRunIndex,
 } from "../forgejo/comment-logic";
@@ -19,7 +18,7 @@ import {
   isEntityContext,
 } from "../github/context";
 import type { ParsedGitHubContext } from "../github/context";
-import { GITHUB_SERVER_URL } from "../github/api/config";
+import { getServerUrl } from "../github/api/config";
 import { checkAndCommitOrDeleteBranch } from "../github/operations/branch-cleanup";
 import { updateClaudeComment } from "../github/operations/comments/update-claude-comment";
 
@@ -54,7 +53,7 @@ export async function updateCommentLink(
   const { owner, repo } = context.repository;
   const platform = detectPlatform();
 
-  const serverUrl = GITHUB_SERVER_URL;
+  const serverUrl = getServerUrl();
   const runIdentifier =
     platform === "forgejo" ? getForgejoRunIndex() : process.env.GITHUB_RUN_ID;
   const jobUrl = `${serverUrl}/${owner}/${repo}/actions/runs/${runIdentifier}`;
@@ -250,10 +249,7 @@ export async function updateCommentLink(
     errorDetails,
   };
 
-  const updatedBody =
-    platform === "forgejo"
-      ? updateForgejoCommentBody(commentInput)
-      : updateCommentBody(commentInput);
+  const updatedBody = updateCommentBody(commentInput);
 
   try {
     await updateClaudeComment(octokit.rest, {