The workflow was passing deleted files to the validation script, which
failed when trying to read them. Add --diff-filter=AMRC to only process
Added, Modified, Renamed, and Copied files.
Adds a GitHub Actions workflow that validates frontmatter in agent,
skill, and command .md files changed by a PR. Checks:
- Agents: name and description are present and parseable
- Skills: description is present (required for Skill tool discovery)
- Commands: description is present and parseable
The workflow only runs when PRs touch files in agents/, skills/, or
commands/ directories, and only validates the changed files.
