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>

.github/workflows/ci.yml

@@ -91,7 +91,7 @@ jobs:
 
       - name: Install frontend dependencies
         working-directory: frontend
-        run: npm install
+        run: npm ci
 
       - name: Frontend theme token policy
         working-directory: frontend
@@ -124,7 +124,7 @@ jobs:
           node-version: "20"
 
       - name: Install docs dependencies
-        run: npm install --prefix docs/website
+        run: npm ci --prefix docs/website
 
       - name: Docs build
         run: npm --prefix docs/website run build

.github/workflows/docs-pages.yml

@@ -32,7 +32,7 @@ jobs:
           node-version: "20"
 
       - name: Install docs dependencies
-        run: npm install --prefix docs/website
+        run: npm ci --prefix docs/website
 
       - name: Build docs site
         run: npm --prefix docs/website run build