Add AI code safety defense system: tests, pre-commit hooks, and CI

Implemented three-layer defense mechanism to prevent AI-generated code from breaking the codebase:
- Layer 1: AGENTS.md with universal AI agent instructions
- Layer 2: Husky pre-commit hooks with test file checking
- Layer 3: GitHub Actions CI with TypeScript, ESLint, tests, and coverage

Includes design document, test infrastructure, and coverage thresholds.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,65 @@
+name: CI
+
+on:
+  push:
+    branches: ['*']
+  pull_request:
+    branches: [main]
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9
+
+      - name: Get pnpm store directory
+        shell: bash
+        run: |
+          echo "STORE_PATH=$(pnpm store path --silent)" >> $GITHUB_ENV
+
+      - name: Setup pnpm cache
+        uses: actions/cache@v4
+        with:
+          path: ${{ env.STORE_PATH }}
+          key: ${{ runner.os }}-pnpm-store-${{ hashFiles('**/pnpm-lock.yaml') }}
+          restore-keys: |
+            ${{ runner.os }}-pnpm-store-
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: TypeScript check
+        run: pnpm typecheck
+
+      - name: ESLint
+        # TODO: Reduce --max-warnings to 0 as pre-existing issues are fixed
+        run: pnpm lint --max-warnings 250
+
+      - name: Prettier check
+        run: pnpm format:check
+
+      - name: Run tests with coverage
+        run: pnpm test:coverage
+
+      - name: Build application
+        run: pnpm build
+
+      - name: Upload coverage report
+        uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: coverage-report
+          path: coverage/
+          retention-days: 7