chore: remove accidentally committed superpowers plan artifact (#126)

## Summary

- Deletes
`docs/superpowers/plans/2026-05-07-full-stack-integration-tests.md` — a
1288-line superpowers planning document that was accidentally bundled
into PR #124
- Planning artifacts from the `docs/superpowers/` tree should never be
committed to the repo

## Test plan

- [ ] Confirm `docs/superpowers/` directory is absent after merge
- [ ] No functional code changed; no tests required

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: CoderCoco