Feature: Notify users when skills are added or updated

Feature Request

Problem

When skills are added or updated in ~/.claude/skills/ or project-level .claude/skills/, users have no way of knowing about the changes unless they manually check the files. This makes it easy to miss new capabilities or improvements.

Proposed Solution

On Claude Code startup (or first prompt of a session), display a notification when skills have changed since the last session:

🆕 New skill available: ui-testing
🔄 Skill updated: claudeception (v1.0.0 → v1.1.0)

Implementation Ideas

1. Cache skill file hashes/timestamps in ~/.claude/.skill-cache
2. On startup, compare current skill files against cache
3. Display diff summary if changes detected
4. Update cache after displaying

Additional Context

• Skills can come from multiple locations (global ~/.claude/skills/, project .claude/skills/, plugins)
• Version info is available in skill YAML frontmatter (version: 1.0.0)
• Could also show brief description of what changed (from git diff or changelog field)

Workaround

Currently possible via UserPromptSubmit hook with a shell script, but native support would be cleaner and more reliable.