[Feature]: Skill Update Notification and Management

## Problem

oh-my-opencode currently has no mechanism for tracking or updating installed skills:

1. **No version awareness** - OmO doesn't know what version of skills are installed
2. **No update notification** - Users manually check GitHub for updates
3. **No centralized management** - Each skill is independently managed

## Current Workflow (Manual)

```bash
# Check for updates manually
cd ~/.config/opencode/skills/ui-ux-pro-max
git fetch && git log HEAD..origin/main --oneline

# Update manually
git pull
```

This is tedious when managing multiple skills (superpowers, planning-with-files, ui-ux-pro-max, etc.)

## Proposed Solution

### 1. Skill Status Command
Add a command or tool to check skill versions:
```
/skill status
# or via tool
find_skills --check-updates
```

Output:
```
Installed Skills:
superpowers (v4.0.3) ✓ up to date
planning-with-files (v2.4.1) ✓ up to date
ui-ux-pro-max (v2.0.0) ⚠ update available (v2.1.0)
```

### 2. Update Notification on Startup
When OmO loads, check if git-based skills have upstream changes:
```
[oh-my-opencode] 1 skill update available: ui-ux-pro-max
```

### 3. Update Command
```
/skill update ui-ux-pro-max
# or update all
/skill update --all
```

## Implementation Notes

- Skills installed via git clone: use `git fetch` + compare HEAD
- Skills as symlinks: follow symlink, check git status
- Track skill metadata in `~/.config/opencode/skill-manifest.json`

## Related Issues

- #930 - OmO skill loading ignores global skills
- #44 - Superpowers integration

## Environment

- oh-my-opencode: 3.0.0-beta.11
- OpenCode: 1.1.28