feat(review): implement fallback chain for review checklists

[Ty-Robb]

Summary

• Created a universal, stack-agnostic default review checklist covering SQL injection, trust boundaries, hardcoded secrets, error handling, and dead code.
• Renamed the existing Rails-specific checklist so it can serve as a reference template for Rails projects.
• Updated the /review skill to dynamically resolve the best checklist via a three-tier fallback chain: configured path in .gstack.json, then project-local custom checklist, then bundled universal default.
• Backwards compatible — existing projects with a custom checklist see zero behavior change.

Test plan

• Verified fallback chain correctly picks up a configured checklist path from .gstack.json.
• Verified fallback to project-local checklist when no config exists.
• Verified final fallback to bundled universal default when neither is present.
  🤖 Authored via Spec-Driven Development