Add CI workflow for markdown linting

[xiapeli]

Summary

Add a GitHub Actions workflow that lints all markdown files on push and pull requests. This ensures consistent formatting across agent definitions, documentation, and templates.

Suggested Approach

1. Create `.github/workflows/lint-markdown.yml`
2. Use markdownlint-cli2 or markdownlint-cli
3. Add a `.markdownlint.json` config file at the repo root with sensible defaults
4. The workflow should:
   • Trigger on push to `main` and on pull requests
   • Lint all `.md` files in the repository
   • Report violations clearly in the PR checks

Suggested Config

```json
{
"default": true,
"MD013": false,
"MD033": false,
"MD041": false
}
```

• MD013 (line length): disabled because agent definitions can have long lines
• MD033 (inline HTML): disabled to allow HTML in templates
• MD041 (first line heading): disabled because YAML frontmatter comes first in templates

Acceptance Criteria

• GitHub Actions workflow file created
• Markdownlint config file created
• Workflow triggers on push to main and on PRs
• All existing markdown files pass linting (fix any violations)
• Workflow runs successfully in CI