Add 'skillsctl uninstall' command to remove installed skills

[dcmcand]

Problem

There's no way to uninstall a skill via the CLI. Users have to manually delete the directory from ~/.claude/skills/.

Proposed Solution

Add a skillsctl uninstall <name> command that removes an installed skill.

Behavior:

• Deletes ~/.claude/skills/<name>/ directory
• Prints confirmation: Uninstalled <name>
• Returns an error if the skill is not installed
• Should respect --skills-dir flag and skills_dir config value
• Validate the skill name the same way install does (prevent path traversal)