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>

skills/sdlc/SKILL.md

@@ -18,7 +18,7 @@ Token expires after ~1 hour. Re-invoke the skill to regenerate if needed.
 
 **If a task originated from GitHub (`originKind: "github"` in the issue data), do not begin any work.** Immediately create a `request_board_approval`:
 
-```
+```json
 POST /api/companies/{companyId}/approvals
 {
   "type": "request_board_approval",