feat: add inspect --installed to show local skill metadata

Summary

skillctl inspect <ref> reads metadata from OCI manifest annotations,
which reflect what was baked in at build time. After install/upgrade,
the local skill.yaml may have a different version (updated from the
tag). There's no way to inspect the installed skill's actual metadata.

Proposed CLI

# Inspect an installed skill by name and target
skillctl inspect document-summarizer --installed --target claude

# Inspect an installed skill in a custom directory
skillctl inspect my-skill --installed -o ~/custom/skills

Motivation

After upgrading document-summarizer from 1.1.0 to 1.2.0, the
remote image's inspect still shows Version: 1.0.0 (the baked-in
metadata). The local skill.yaml correctly shows 1.2.0. Users
need a way to inspect the installed state.

Context

Discovered while testing the upgrade workflow (PR #31). The local
skill.yaml is the authoritative source for installed skill
metadata after install/upgrade.

Use the existing skillctl inspect command as the entry point and trace how an installed skill's local skill.yaml is located, including the target and custom -o directory. Done means inspect --installed reports the installed metadata and supports both the named target and custom directory examples.