ci: align source checks and agent skills

[aoirint]

Warning

This pull request was created with assistance from LLMs.

Summary

• deploy aoirint/skills at 6a1e6431
• use the shared Markdown configuration owned by the GitHub Actions Skill
• complete Composite Action portability and preserve singular Check and Test contexts

The final portability corrections are reviewed in aoirint/skills#90.

Testing

• APM 0.27.0 lock, frozen install, and audit (10/10)
• actionlint where workflows exist
• git diff --check