epic: convert /plan into agent-first CLI package

[Delqhi]

Summary

Transform /plan from a docs-heavy skill repo into an agent-first enterprise CLI package with deterministic JSON output, strict validation, and governance-aware execution.

Workstreams

• feat(plan-cli): scaffold strict JSON-first CLI package #3 — CLI scaffold and JSON contract
• feat(plan-core): port validator, DAG, and Monte Carlo engine #5 — core validation / DAG / Monte Carlo engine
• feat(plan-governance): add audit, approval, rollback, and drift controls #6 — governance, audit, rollback, and drift controls
• chore(plan-ci): add packaging, tests, and release automation #2 — packaging, CI, tests, and release automation
• docs(plan): rewrite README and migration guide for agent-first transition #4 — README and migration guide

Current reusable assets

• SKILL.md
• BEST_PRACTICES.md
• templates/*
• scripts/plan_validate.py
• scripts/dag_builder.py
• scripts/monte_carlo.py

Target outcome

• opencode plan exists as a deterministic CLI
• every command emits machine-readable output
• repo is packaged, tested, and release-ready
• legacy skill guidance stays aligned during migration