chore(tooling): add Claude Code slash commands for housekeeping and security alerts (#3251)

Ports two Claude Code slash commands from the nimbus repo, tailored to ui-kit:

- /housekeeping — update pnpm-workspace.yaml catalog entries to latest
  minor/patch versions in risk-ordered cohorts, with validation between
  checkpoints. Accepts natural-language scope filters.
- /github/fix-security-alerts — triage and fix Dependabot security alerts,
  with auto-fixable updates, validation, and optional PR creation.

Author: misama-ct