Cleanup

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

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

<execution_context> @~/.claude/get-shit-done/workflows/cleanup.md </execution_context>

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