Add Claude-led orchestration skill (Plan → Tasks → Execute → Review) with docs and contract test

[Copilot]

This PR adds an explicit orchestration contract for the main Claude thread to run end-to-end workflow in the intended “Claude plans/reviews, Codex executes” model. It fills the current gap between existing /codex:* commands and internal single-shot skills while preserving codex:codex-rescue as a forward-only subagent.

• New orchestration skill

  • Added plugins/codex/skills/codex-orchestration/SKILL.md as a user-invocable skill.
  • Defines the four phases:
    • PLAN: architecture decisions, target files, constraints, edge cases, implementation brief.
    • TASKS: small bounded tasks with explicit acceptance criteria.
    • EXECUTE: delegate via /codex:rescue (prefer --background for long work), monitor with /codex:status, collect via /codex:result, continue with --resume.
    • REVIEW: validate against brief, optionally use /codex:review or /codex:adversarial-review, loop back through /codex:rescue --resume until passing.
  • Includes role boundaries: Claude does not write production code in this mode; Codex does not own design decisions.

• README flow documentation

  • Added a concise section in Typical Flows: Orchestration: Plan → Tasks → Execute → Review.
  • Documents the command-level loop and how to iterate safely across background runs and review cycles.

• Test coverage for orchestration contract

  • Added tests/orchestration.test.mjs.
  • Verifies:
    • new skill file exists at expected path,
    • frontmatter (name, user-invocable) is correct,
    • all four workflow phases are present,
    • required command markers are present,
    • forwarder constraint is explicit (codex-rescue remains single-shot forwarder).

Example contract excerpt:

## EXECUTE

- Delegate each task with `/codex:rescue` (write-capable by default).
- Prefer `/codex:rescue --background` for long-running or open-ended work.
- Track background jobs with `/codex:status` and collect outputs with `/codex:result`.
- Send follow-ups in the same Codex thread using `/codex:rescue --resume` when the direction is iterative.