feat: add remove command to uninstall skills by name

[thanatosartcoder]

Summary

• Add autoskills remove <skill> command to uninstall installed skills
• Add autoskills rm <skill> alias for the remove command
• Add interactive mode with autoskills remove to select skills to remove
• Add confirmation prompt before removing skills
• Add --dry-run support for the remove command
• Add informative message when skill is not installed
• Fix getInstalledSkillNames to properly detect installed skills when lockfile has empty skills object
• Fix multiSelect initialSelected array length validation
• Handle non-interactive stdin in interactive remove mode

Usage

autoskills remove <skill-name>     # Remove specific skill
autoskills rm <skill-name>        # Alias for remove
autoskills remove                 # Interactive mode (select skills)
autoskills remove <skill> --dry-run  # Preview what would be removed
autoskills remove <skill> -y       # Remove without confirmation

Behavior

• Removes canonical skill directory: .agents/skills/<skill>
• Removes symlinks from all agent folders: .claude/skills/, .cursor/skills/, etc.
• Updates skills-lock.json to remove the skill entry
• Asks for confirmation before removing (unless -y flag is used)
• Shows informative message if skill is not installed