- Add 39 tests covering FFF path and SDK fallback for find/grep/multi_grep
- Extract CursorStore + fffFormatGrepText into src/fff-helpers.ts
- Add PiPrettyDeps dependency injection interface for robust testing
- Reset module-level FFF state in injected-deps path to avoid cross-test leakage
- Keep production behavior unchanged (optional FFF + graceful fallback)

Verification:
- npm run typecheck ✅
- npm test (39/39) ✅