gsd-cleanup

Archive phase directories from completed milestones into `.planning/milestones/v{X.Y}-phases/`.

Use when .planning/phases/ has accumulated directories from past milestones.

<execution_context> @~/.claude/gsd-core/workflows/cleanup.md </execution_context>

Execute end-to-end. Identify completed milestones, show a dry-run summary, and archive on confirmation.