chore: remove stale build artifacts, fix gitignore, use npm ci in CI

- Remove 114 tracked build/lib/ files (stale setuptools artifact that
  broke the repo-hygiene CI guard)
- Add build/ to .gitignore
- Remove AGENTS.MD from .gitignore (track agents.md instead)
- Delete ad-hoc scripts: diag_pubs.py, test_enrich.py
- Delete orphaned frontend/docs/website/package-lock.json
- Replace npm install with npm ci in ci.yml and docs-pages.yml for
  deterministic lockfile installs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -12,9 +12,9 @@ htmlcov/
 *.log
 .DS_Store
 planning/
+build/
 frontend/node_modules/
 frontend/dist/
 frontend/.vite/
 docs/website/node_modules/
 docs/website/build/
-AGENTS.MD