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/platform/detector.ts

@@ -6,24 +6,47 @@
  * 2. FORGEJO env var (set by Forgejo runner v7.0+)
  * 3. GITHUB_SERVER_URL heuristic (non-github.com = Forgejo)
  * 4. Default to GitHub
+ *
+ * The result is memoized after first detection — platform doesn't change
+ * at runtime and this avoids redundant env reads across 12+ call sites.
  */
 
 export type Platform = "github" | "forgejo";
 
+let _cachedPlatform: Platform | undefined;
+
 export function detectPlatform(): Platform {
+  if (_cachedPlatform !== undefined) return _cachedPlatform;
+
+  let result: Platform = "github";
+
   // 1. Explicit platform from action.yml detect step or user input
   const platformEnv = process.env.PLATFORM;
-  if (platformEnv === "forgejo") return "forgejo";
-  if (platformEnv === "github") return "github";
-
+  if (platformEnv === "forgejo") {
+    result = "forgejo";
+  } else if (platformEnv === "github") {
+    result = "github";
+  }
   // 2. FORGEJO env var (set by Forgejo runner v7.0+)
-  if (process.env.FORGEJO === "true") return "forgejo";
-
+  else if (process.env.FORGEJO === "true") {
+    result = "forgejo";
+  }
   // 3. GITHUB_SERVER_URL heuristic — if not github.com, likely Forgejo/Gitea
-  const serverUrl = process.env.GITHUB_SERVER_URL || "";
-  if (serverUrl && !serverUrl.includes("github.com")) {
-    return "forgejo";
+  else {
+    const serverUrl = process.env.GITHUB_SERVER_URL || "";
+    if (serverUrl && !serverUrl.includes("github.com")) {
+      result = "forgejo";
+    }
   }
 
-  return "github";
+  _cachedPlatform = result;
+  return result;
+}
+
+/**
+ * Reset the memoized platform detection result.
+ * Only needed in tests that change platform-related env vars between cases.
+ */
+export function _resetPlatformCache(): void {
+  _cachedPlatform = undefined;
 }