Add Ghostable adoption prompt command and setup integration

[joerucci]

Summary

• Add a new ghostable adopt command that generates a structured adoption prompt for AI-assisted Ghostable onboarding.
• Support interactive section selection, --yes, --all, --sections, and optional CI guidance.
• Extend ghostable setup with an opt-in adoption prompt flow and add help text for the new command.
• Cover the new command and setup integration with tests, including guardrail checks for safe output.

Testing

• Added unit tests for adopt selection modes, prompt content, and safety constraints.
• Added setup tests for explicit prompt printing, interactive section selection, and non-interactive defaults.
• Not run (not requested).