perf: reduce idle desktop load by gating plan-mode checks

- Add startup feature flag for plan mode from OPENCODE_EXPERIMENTAL* env vars.
- Stop recurring app health polling and keep a one-time startup check.
- Skip plan-mode synthetic parsing and plan tab/file work when flag is off.

packages/ui/src/stores/useFeatureFlagsStore.ts

@@ -0,0 +1,11 @@
+import { create } from 'zustand';
+
+type FeatureFlagsStore = {
+  planModeEnabled: boolean;
+  setPlanModeEnabled: (enabled: boolean) => void;
+};
+
+export const useFeatureFlagsStore = create<FeatureFlagsStore>((set) => ({
+  planModeEnabled: false,
+  setPlanModeEnabled: (enabled) => set({ planModeEnabled: enabled }),
+}));