Implement PlanAnvil skill

[KeyffMS]

Summary

Implements the authoritative PlanAnvil 2.1 contract from docs/ as a concise, explicit-only Codex skill with deterministic supporting tooling.

Included

• concise .agents/skills/plan-anvil/SKILL.md controller;
• lifecycle, plan, artifact, review, and execution references;
• versioned JSON Schemas and canonical templates;
• Python 3.11 standard-library scripts for preflight, reversible Git capability probing, isolated worktree creation, repository profiling, instruction mapping, run scaffolding, validation, blind review, controlled commit, and final stop;
• optional read-only Codex profiler/reviewer agents and defense-in-depth hooks;
• golden contract examples and capability-evidence scaffolding;
• root repository instructions, release notes, and updated README.

Validation

• python -m unittest discover -s .agents/skills/plan-anvil/tests -v — 22 tests passed;
• python -m compileall -q .agents/skills/plan-anvil .codex/hooks — passed;
• archive source and decoded payload SHA-256 verification — passed;
• canonical JSON, UTF-8/LF, and git diff --check validation — passed.

Result

The final implementation is committed as cc5393b0da5d5a73316fb19f480040361382064e. Temporary .bootstrap files and the materialization workflow were removed from the final tree.

Production readiness remains gated on committed, sanitized REPRODUCED Codex capability evidence defined in docs/CODEX_CAPABILITY_BASELINE.md.