Fix grep portability bug and document doc-drift script in CLAUDE.md

- Use grep -rE (ERE) instead of BRE \| alternation for BSD/macOS compat
- Add doc-drift section to CLAUDE.md Testing section with usage and purpose

Nightshift-Task: doc-drift
Nightshift-Ref: https://github.com/marcus/nightshift

CLAUDE.md

@@ -77,6 +77,12 @@ cd client && npm run test:watch
 - Export pure functions (validators, formatters, etc.) for direct testing
 - Run `npm test` in both `server/` and `client/` before committing
 
+**Doc drift check:**
+```bash
+node scripts/doc-drift.js
+```
+Scans `CLAUDE.md` and `PRD.md` for verifiable code references (file paths, API routes, component names) and cross-checks each against the filesystem and source tree. Prints a PASS/FAIL report with doc name and line number. Exits non-zero on any failure — suitable for CI gating.
+
 ## Application Structure
 
 The default route `/` renders the paycheck-centric main view (`client/src/pages/PaycheckView.jsx`). It shows the current month's two paychecks side-by-side with bills, paid status, one-time expenses, and remaining balance. Month navigation (prev/next) fetches data via `GET /api/paychecks?year=&month=`.