fix(skill-installer): add --force flag to allow reinstall/update

[skylarkoo7]

Summary

Fixes #127.

The skill installer currently aborts with a hard error when the destination directory already exists. There is no built-in way to update a skill to a newer version, reinstall after local corruption, or recover from a partially-failed installation. This PR adds a --force flag that removes the existing directory before copying the new version in.

Changes

scripts/install-skill-from-github.py

• Added --force CLI flag (defaults to False, preserving the current safe default).
• Updated _copy_skill() to accept a force parameter; when True the existing directory is removed via shutil.rmtree before copytree.
• Updated the early exists-check in main() to respect --force and surface an actionable hint ("Use --force to remove the existing skill and reinstall") when the flag is absent.
• Success message now says "Reinstalled" vs "Installed" when --force is active.

scripts/list-skills.py

• Added _installed_skills_detail() to return installed skill paths and health status (detects missing SKILL.md as "broken").
• Added --show-path CLI flag that displays the install directory and health status for each installed skill in both text and JSON output modes.

SKILL.md

• Documented --force in the Scripts and Behavior and Options sections.
• Documented --show-path in the Scripts section.
• Updated the Options line to include --force.

Test plan

• Install a fresh skill, verify normal behavior unchanged
• Install the same skill again without --force, verify error message includes the --force hint
• Install the same skill again with --force, verify it reinstalls and prints "Reinstalled"
• Run list-skills.py --show-path and verify paths and status are shown
• Create a broken skill dir (no SKILL.md), verify --show-path reports "broken"
• Verify --show-path --format json includes path and status fields