docs: trim CLAUDE.md to gotchas-only, add CONTRIBUTING.md
CLAUDE.md's stated purpose is "common mistakes and confusion points."
Several sections had grown into general style guides and process docs
that don't serve that purpose:
- Remove generic testing workflow, quality standards, coverage expectations
→ moved to CONTRIBUTING.md
- Remove defensive programming boilerplate, global error handler template,
error testing examples → discoverable from existing code
- Remove security test coverage requirements and security checklist
→ moved to CONTRIBUTING.md
- Remove Bruno file template and testing workflow → moved to CONTRIBUTING.md
Bug fixes in CLAUDE.md:
- Fix stale `role.permissions.includes()` example → role_permissions join table
- Add `forum.board` to lexicon record ownership list (added by ATB-23)
- Add docs/plans/complete/ convention
CONTRIBUTING.md is a new contributor-facing document covering testing
workflow, error handling patterns (with examples), security checklist,
and Bruno template — content humans read before their first PR.