feat: add /dogfood skill for automated release validation

10-phase dogfooding workflow: install from npm, cold-start testing,
full command sweep with flag coverage, rebuild/staleness detection,
engine comparison, changelog-targeted tests, thinking space for
coverage gaps, auto-file bugs as GitHub issues with fix PRs, and
structured report generation. Self-improving — the skill encourages
the agent to update its own instructions.

Usage: /dogfood <version>

Author: github-actions[bot]