feat(FN-705): add rate-limit retry with exponential backoff for AI agents

- Add rate-limit retry utility with exponential backoff, jitter, and configurable limits
- Integrate retry logic into executor, triage, and merger agent sessions
- Export retryOnRateLimit from engine package index
- Remove unused DirectoryPicker component, stale CSS, and dead dashboard routes
- Update SetupWizardModal and usage tests to reflect cleanup

packages/engine/src/merger.test.ts

@@ -14,6 +14,10 @@ vi.mock("node:fs", () => ({
   readFileSync: vi.fn(),
 }));
 
+vi.mock("./rate-limit-retry.js", () => ({
+  withRateLimitRetry: (fn: () => Promise<any>) => fn(),
+}));
+
 import {
   aiMergeTask,
   findWorktreeUser,