feat(quality): add markdownlint-cli2 to quality checks (#1039)

- Add markdownlint-cli2 as dev dependency
- Add lint:md script to package.json
- Add markdownlint job to CI workflow
- Configure 5 rules: heading-increment, no-duplicate-heading,
  no-trailing-punctuation, no-bare-urls, no-space-in-emphasis
- Fix existing violations across 19 markdown files
- No auto-fix to prevent destructive changes

Closes #1034

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Brian <bmadcode@gmail.com>

.github/workflows/quality.yaml

@@ -3,6 +3,7 @@ name: Quality & Validation
 # Runs comprehensive quality checks on all PRs:
 # - Prettier (formatting)
 # - ESLint (linting)
+# - markdownlint (markdown quality)
 # - Schema validation (YAML structure)
 # - Agent schema tests (fixture-based validation)
 # - Installation component tests (compilation)
@@ -50,6 +51,24 @@ jobs:
       - name: ESLint
         run: npm run lint
 
+  markdownlint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version-file: ".nvmrc"
+          cache: "npm"
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: markdownlint
+        run: npm run lint:md
+
   validate:
     runs-on: ubuntu-latest
     steps: