chore: pause work and save context handoff

.agent/skills/gsd-cleanup/SKILL.md

@@ -0,0 +1,19 @@
+---
+name: gsd-cleanup
+description: Archive accumulated phase directories from completed milestones
+---
+
+<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>
+@.agent/get-shit-done/workflows/cleanup.md
+</execution_context>
+
+<process>
+Follow the cleanup workflow at @.agent/get-shit-done/workflows/cleanup.md.
+Identify completed milestones, show a dry-run summary, and archive on confirmation.
+</process>