M12 Final Run Readiness Layer

Added:
- CODEX_START.md
- docs/20-run-readiness/RUN_READINESS_FINAL.md
- docs/20-run-readiness/PASS_SEQUENCE_FINAL.md
- docs/20-run-readiness/CODEX_TOMORROW_RUNBOOK.md
- docs/20-run-readiness/IMPLEMENTATION_READINESS_CHECKLIST.md
- docs/20-run-readiness/REPO_CONFLICT_RESOLUTION.md
- prompts/codex/final/DAY_1_MASTER_PROMPT.md
- prompts/codex/final/PASS_00_REPO_AUDIT_FINAL.md
- prompts/codex/final/PASS_01_BACKEND_FOUNDATION_FINAL.md
- prompts/codex/final/PASS_02_DOMAIN_DATABASE_FINAL.md
- scripts/preimplementation_audit.py
- scripts/run_readiness_check.sh
- Makefile
- RELEASE_NOTES/v0.12-m12-final-run-readiness.md

Changed:
- README.md