chore(plan-ci): add packaging, tests, and release automation

[Delqhi]

Scope

Add the package scaffolding needed to expose opencode plan as a strict JSON-first CLI.

Build requirements

• pyproject.toml and package metadata
• Typer-based entrypoint under src/plan_cli/
• commands: init, validate, simulate, execute, audit, rollback
• Pydantic v2 response models and JSON Schema export
• stable exit codes and agent_hint response fields

Acceptance criteria

• --help shows the full command tree
• init can create a starter plan from a template
• every command supports machine-readable output
• package builds cleanly for install and release