fix: prevent OOM, orphaned agents, and unbounded growth during overnight builds

- Move handleStartFollowupReview after runningReviews.has() duplicate check to
  prevent XState actor getting stuck in reviewing state when follow-up is already
  running (mirroring the regular review handler pattern)
- Add CLEAR_REVIEW handler to reviewing state so handleClearReview works correctly
  when an active review is cleared
- Remove dead-code START_REVIEW transition and always-false isNotAlreadyReviewing
  guard from reviewing state — duplicate prevention is handled at the IPC layer

Co-Authored-By: Claude Opus 4.6 <[email protected]>

apps/frontend/src/main/ipc-handlers/github/pr-handlers.ts

@@ -2901,20 +2901,26 @@ export function registerPRHandlers(getMainWindow: () => BrowserWindow | null): v
         return;
       }
 
-      // Get previous result for followup context
-      const previousResult = await withProjectOrNull(projectId, async (project) => {
-        return getReviewResult(project, prNumber) ?? undefined;
-      }) ?? undefined;
-
-      // Notify state manager that followup review is starting
-      prReviewStateManager.handleStartFollowupReview(projectId, prNumber, previousResult);
-
       try {
         await withProjectOrNull(projectId, async (project) => {
           const sendProgress = (progress: PRReviewProgress): void => {
             prReviewStateManager.handleProgress(projectId, prNumber, progress);
           };
 
+          const reviewKey = getReviewKey(projectId, prNumber);
+
+          // Check if already running — before notifying state manager to avoid stuck reviewing state
+          if (runningReviews.has(reviewKey)) {
+            debugLog("Follow-up review already running", { reviewKey });
+            return;
+          }
+
+          // Get previous result for followup context
+          const previousResult = getReviewResult(project, prNumber) ?? undefined;
+
+          // Notify state manager that followup review is starting (after duplicate check)
+          prReviewStateManager.handleStartFollowupReview(projectId, prNumber, previousResult);
+
           // Comprehensive validation of GitHub module
           const validation = await validateGitHubModule(project);
           if (!validation.valid) {
@@ -2923,13 +2929,6 @@ export function registerPRHandlers(getMainWindow: () => BrowserWindow | null): v
           }
 
           const backendPath = validation.backendPath!;
-          const reviewKey = getReviewKey(projectId, prNumber);
-
-          // Check if already running
-          if (runningReviews.has(reviewKey)) {
-            debugLog("Follow-up review already running", { reviewKey });
-            return;
-          }
 
           // Register as running BEFORE CI wait to prevent race conditions
           // Use CI_WAIT_PLACEHOLDER sentinel until real process is spawned

apps/frontend/src/shared/state-machines/__tests__/pr-review-machine.test.ts

@@ -193,7 +193,7 @@ describe('prReviewMachine', () => {
     });
   });
 
-  describe('guard: reject START_REVIEW when already reviewing', () => {
+  describe('reject START_REVIEW when already reviewing', () => {
     it('should stay in reviewing when START_REVIEW is sent again', () => {
       const snapshot = runEvents([
         { type: 'START_REVIEW', prNumber: 42, projectId: 'proj-1' },

apps/frontend/src/shared/state-machines/pr-review-machine.ts

@@ -59,10 +59,6 @@ export const prReviewMachine = createMachine(
       },
       reviewing: {
         on: {
-          START_REVIEW: {
-            target: 'reviewing',
-            guard: 'isNotAlreadyReviewing',
-          },
           SET_PROGRESS: {
             actions: 'setProgress',
           },
@@ -78,6 +74,10 @@ export const prReviewMachine = createMachine(
             target: 'error',
             actions: 'setCancelledError',
           },
+          CLEAR_REVIEW: {
+            target: 'idle',
+            actions: 'clearContext',
+          },
           DETECT_EXTERNAL_REVIEW: {
             target: 'externalReview',
             actions: 'setExternalReview',
@@ -142,9 +142,6 @@ export const prReviewMachine = createMachine(
     },
   },
   {
-    guards: {
-      isNotAlreadyReviewing: () => false,
-    },
     actions: {
       setReviewStart: assign({
         prNumber: ({ event }) => (event as { prNumber: number }).prNumber,