refine(skills): openmetadata-user - fix naming (2 segments, invalid focus)

## Summary

`openmetadata-user` has 2 segments and uses invalid focus 'user'.

## Current Name Analysis

- **Name**: openmetadata-user
- **Segments**: 2 (openmetadata, user)
- **Focus**: user ✗ (not in: ops, dev, eng, nub, xec)
- **Issues**: Missing category prefix + invalid focus

## Refinement Checklist

- [ ] Determine appropriate category and focus
- [ ] Option A: Rename to `data-openmetadata-nub` (newcomer/end-user focus)
- [ ] Option B: Rename to `data-openmetadata-guide-dev` (user guide for developers)
- [ ] Option C: Add 'user' to valid focus list (requires justfile update)
- [ ] If renaming, update skill directory name
- [ ] If renaming, update SKILL.md frontmatter
- [ ] Run `just validate-skill components/skills/` to verify
- [ ] Update any references to this skill in other files

## Allowed Focus Values

- `ops` - Operations/SRE focus
- `dev` - Development focus
- `eng` - Engineering/Architecture focus
- `nub` - Newcomer/beginner focus (closest to 'user')
- `xec` - Executive/management focus

## Validation Command

```bash
just validate-skill components/skills/openmetadata-user
```