Reconcile installed.yaml with on-disk skill installs when listing

[hbelmiro]

Problem

skill list --installed reads installed.yaml only. If users delete skill files manually, the list diverges from the filesystem.

Proposal

• When listing installed skills, verify each entry’s target directory and artifact.json.
• Repair: If install is missing but cache is valid, re-copy from cache (InstallToTarget).
• If cache is missing, mark entry error (or remove — product choice) and persist installed.yaml.
• Document that --installed may update installed.yaml / disk.

Acceptance criteria

• Tests: missing target dir + good cache → repaired; missing cache → error state persisted.