Add 'skillsctl list' command to show installed skills

[dcmcand]

Problem

There's no way to see what skills are currently installed locally. Users have to manually check ~/.claude/skills/ to see what's there.

Proposed Solution

Add a skillsctl list command that lists all installed skills in the configured skills directory.

Output should show:

• Skill name (derived from directory name)
• File path

Example:

$ skillsctl list
NAME              PATH
review-iac        ~/.claude/skills/review-iac/SKILL.md
skillsctl-usage   ~/.claude/skills/skillsctl-usage/SKILL.md

Should respect the --skills-dir flag and skills_dir config value.