skill --install: explain each action with reasons and path abbreviation

[dreikanter]

Summary

Improve the plain-text output of skill --install to be more informative and user-friendly:

• Each action line now includes the reason it was chosen (e.g., "already up to date" for skip, "edited locally, pass --force to replace" for conflict)
• Home directory paths are abbreviated to ~ for better readability
• Actions are padded to consistent column widths using tabwriter for alignment
• Error messages are now presented as an "error" action with the message as the reason
• --json output remains unchanged

The plain-text format now reads: agent action path (reason), making it clearer why each file was or wasn't installed.

Checklist

• Updated CHANGELOG.md under Unreleased