M13 — Codex Optimization Pack

Purpose:
- Improve Codex execution quality after M12 final run readiness.
- Add reusable skills, prompt discipline, token policy, secrets policy, parallel agent strategy and pass completion prompts.

Added:
- docs/30-codex-optimization/CODEX_OPTIMIZATION_OVERVIEW.md
- docs/30-codex-optimization/CODEX_RUN_CHECKLIST.md
- docs/30-codex-optimization/PROMPT_DISCIPLINE.md
- docs/30-codex-optimization/TOKEN_BUDGET_POLICY.md
- docs/30-codex-optimization/SECRETS_AND_ENV_POLICY.md
- docs/30-codex-optimization/PARALLEL_AGENT_STRATEGY.md
- docs/30-codex-optimization/CODEX_SKILLS_INDEX.md
- docs/30-codex-optimization/M13_HANDOFF_SUMMARY.md
- skills/*/SKILL.md
- prompts/codex/m13/*.md
- scripts/validate_m13_codex_assets.py

Updated:
- README.md
- CODEX_START.md
- Makefile
- scripts/run_readiness_check.sh
- CHANGELOG.md