Create Plan Artifact

Generate .agent/{branch-name}/plan.md — the single source of truth for autonomous execution.

A new Claude session MUST be able to execute from this plan alone without the original conversation.

Prerequisites

Before invoking this skill:

1. Phase 0 (Validation) must be complete — requirements confirmed with user
2. Phase 1 (Planning) must be complete — codebase analyzed, decisions made
3. Confidence gate should have passed (90%+ on plan mode)
4. A worktree must exist — plan.md is created INSIDE the worktree, never on main