feat(ci): add markdownlint-cli2 for consistent markdown formatting

Add automated markdown linting to ensure consistent formatting across
all markdown files in the repository.

Changes:
- Add .markdownlint-cli2.jsonc configuration
- Create .github/workflows/lint.yml for CI/CD integration
- Fix all 908 existing markdown errors across 27 files
- Enforce ATX-style headings and asterisk emphasis
- Set consistent 2-space list indentation

This prevents markdown errors after project initialization and
maintains high documentation quality standards.

.github/workflows/lint.yml

@@ -0,0 +1,18 @@
+name: Lint
+
+on:
+  push:
+    branches: ["main"]
+  pull_request:
+
+jobs:
+  markdownlint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Run markdownlint-cli2
+        uses: DavidAnson/markdownlint-cli2-action@v19
+        with:
+          globs: '**/*.md'