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

scripts/doc-drift.js

@@ -79,7 +79,7 @@ function apiRouteExists(routePath) {
   const fragment = clean.replace(/^\/api/, '');
   try {
     const out = execSync(
-      `grep -r --include="*.js" -l "${clean}\\|${fragment}" "${path.join(ROOT, 'server/src/routes')}"`,
+      `grep -rE --include="*.js" -l "${clean}|${fragment}" "${path.join(ROOT, 'server/src/routes')}"`,
       { stdio: ['pipe', 'pipe', 'pipe'] }
     ).toString().trim();
     return out.length > 0;