feat(version): factory-health (or a new preflight) should surface plugin version + check freshness vs latest release

## Summary

The factory pipeline depends on the installed vsdd-factory plugin version, which is cached in `~/.claude/plugins/cache/claude-mp/vsdd-factory//`. Today there is no surface to:

- Tell the operator which plugin version is installed
- Compare against the latest release
- Warn about staleness
- Detect mid-pipeline version drift

## Proposed

`/vsdd-factory:factory-health` (or new `/vsdd-factory:plugin-version`) should report:

```
Plugin: vsdd-factory@1.0.0-rc.21
Installed: 2026-06-22 (1 day ago)
Latest: 1.0.0-rc.25 (2026-06-22) ← 4 versions newer
Update: claude-mp update vsdd-factory
```

Implementation sketch:

```bash
installed=$(jq -r '.version' "${CLAUDE_PLUGIN_ROOT}/.claude-plugin/plugin.json")
latest=$(gh api repos/drbothen/vsdd-factory/releases/latest --jq '.tag_name' | sed 's/^v//')
[ "$installed" = "$latest" ] || warn "newer release available: $latest"
```

## Acceptance criteria

- [ ] Plugin version surfaced at session start.
- [ ] Latest release compared (offline-tolerant — skip if gh unreachable).
- [ ] Staleness warning printed when behind by ≥1 release.
- [ ] Hooked into `/vsdd-factory:factory-health` or a dedicated skill.

## Found during

`/vsdd-factory:setup-env` on `switchboard-blue` (2026-06-23, vsdd-factory@1.0.0-rc.21). No surface displayed the plugin version; I had to `cat .claude-plugin/plugin.json` manually to learn it. Operators running long pipelines should know whether the plugin updated mid-session.