refactor: remove plan polling, add message-driven detection via usePlanDetection hook

packages/ui/src/hooks/usePlanDetection.ts

@@ -0,0 +1,48 @@
+import React from 'react';
+import { useSessionMessages } from '@/sync/sync-context';
+import { getSyncParts } from '@/sync/sync-refs';
+import { useSessionUIStore } from '@/sync/session-ui-store';
+import { useFeatureFlagsStore } from '@/stores/useFeatureFlagsStore';
+
+/**
+ * Watches session messages for plan creation and marks sessions as plan-available.
+ * 
+ * This is the single source of truth for plan detection. When a plan_enter tool
+ * executes, it creates a synthetic message like "The plan at ${path}" or 
+ * "User has requested to enter plan mode". We detect these and signal availability.
+ * 
+ * The Header component subscribes to sessionPlanAvailable map to show/hide the Plan tab.
+ */
+export const usePlanDetection = (sessionId: string, directory?: string) => {
+  const planModeEnabled = useFeatureFlagsStore((state) => state.planModeEnabled);
+  const markSessionPlanAvailable = useSessionUIStore((state) => state.markSessionPlanAvailable);
+  const isSessionPlanAvailable = useSessionUIStore((state) => state.isSessionPlanAvailable);
+  const messages = useSessionMessages(sessionId, directory);
+
+  React.useEffect(() => {
+    // Early exit if plan mode is disabled - don't parse messages
+    if (!planModeEnabled) return;
+    if (!sessionId) return;
+
+    // Already marked as available - no need to check again
+    if (isSessionPlanAvailable(sessionId)) return;
+
+    // Scan messages for plan references
+    for (const message of messages) {
+      // Only check assistant messages for plan references
+      if (message.role !== 'assistant') continue;
+
+      const parts = getSyncParts(message.id, directory);
+      for (const part of parts) {
+        if (part.type !== 'text') continue;
+        const text = (part as { text?: string }).text || '';
+
+        // Check for plan file reference in synthetic messages
+        if (text.includes('The plan at ') || text.includes('User has requested to enter plan mode')) {
+          markSessionPlanAvailable(sessionId);
+          return;
+        }
+      }
+    }
+  }, [planModeEnabled, sessionId, directory, messages, markSessionPlanAvailable, isSessionPlanAvailable]);
+};