fix: restore CI workflow with markdownlint config

- Restore .github/workflows/ci.yaml that was deleted in April cleanup
- Add .markdownlint.yaml with relaxed rules for skill files
- Fix MD040 error in skills/sdlc/SKILL.md (add language to code block)
- Allows line lengths > 80, emphasis-as-headings, compact tables

Fixes CI failures on 'Merge POLICIES.md content into agent instruction bundles' commit.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci.yaml

@@ -0,0 +1,17 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Lint Markdown
+        uses: DavidAnson/markdownlint-cli2-action@v19
+        with:
+          globs: "**/*.md"