Improvement: Add cleanup prompt for superpower-generated artifacts

[smitropoulos]

When finishing a development branch using the 'finishing-a-development-branch' skill, the AI should prompt the user to clean up any superpower-specific artifacts (e.g., docs/superpowers/specs, plans, or .bak files) created during the session.

I've implemented a local solution by adding a Step 6 to the 'finishing-a-development-branch/SKILL.md' file:

Step 6: Cleanup Superpowers Artifacts

For all options:

After executing the chosen option, ask if the user wants to clean up any superpowers artifacts created during the session:

Would you like to remove any superpowers artifacts (e.g., docs/superpowers/specs, plans, or .bak files) created during this task? (y/n)

If the user confirms, identify and remove the files (using rm), then confirm deletion.

This would help keep the user's workspace clean and prevent stale documentation or plan files from accumulating.

[obra]

The dated specs and plans are important artifacts that explain how and why things got built. I feel pretty strongly that you should be hanging on to them for later.

[marcindulak]

A similar question got asked at Fission-AI/OpenSpec#878.