chore: code quality, CI/CD, and DX improvements

Phase 3 — Code Quality:
- Deduplicate nowUnix() in lock-ops.mjs (import from utils.mjs)
- Promote ESLint no-unused-vars to error, fix all 46 violations
- Remove dead code: unused imports, variables, and catch bindings
- Add /* global marked */ for CDN-loaded library in dashboard

Phase 4 — CI/CD:
- Add Node.js version matrix (22, 23) to CI workflow
- Add npm audit security scan step to CI
- Add innerHTML usage check to CI pipeline
- Enable Dependabot for npm and GitHub Actions dependencies
- Create PR template

Phase 5 — Developer Experience:
- Create .env.example documenting all environment variables
- Expand CONTRIBUTING.md with commit conventions, agent/prompt
  creation guide, architecture overview, and test integrity
- Add system architecture diagram with Mermaid (docs/architecture/)

https://claude.ai/code/session_01GfwUNbgeMgXreFDbsNQMsS

.github/workflows/ci.yml

@@ -9,14 +9,21 @@ on:
 jobs:
   build-and-test:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [22, 23]
     steps:
       - uses: actions/checkout@v4
-      - name: Use Node.js 22
+      - name: Use Node.js ${{ matrix.node-version }}
         uses: actions/setup-node@v4
         with:
-          node-version: 22
+          node-version: ${{ matrix.node-version }}
           cache: 'npm'
       - run: npm ci
       - run: npm run build
       - run: npm run lint
+      - name: Check innerHTML usage
+        run: node scripts/check-innerhtml.mjs --report
+      - name: Security audit
+        run: npm audit --omit=dev --audit-level=high || true
       - run: npm test