docs: reference shared constitution live, fix spec-kit refs

Reduce .specify/memory/constitution.md to a pointer that reads the canonical ai-resources constitution live via glab instead of vendoring its content. Move MakeRails-specific principles to AGENTS.md "Project Principles". Repoint dangling "Constitution Principle IV" references in specs/ to the named Test Coverage principle to avoid collision with the remote constitution's own Principle IV.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com