Create the plan artifact (plan.md + checks.yaml executable acceptance checks) in `.agent/{branch}/` from the current conversation context. Captures all Phase 0-1 discussion into a structured, self-contained document that enables context recovery and session handoff. On every invocation, writes plan.md and re-derives checks.yaml from the Acceptance Criteria; an immutable plan.vN.md snapshot is written only in opt-in snapshot mode (`Skill("aw-create-plan", "snapshot")`). Use after planning is complete and confidence gate passes — and again on every plan iteration (user-requested refinement or Phase 4 auto-replan). Triggers on create plan, generate plan, write plan artifact, regenerate plan, iterate on plan.