docs: archive M11 completion (drag-out) to COMPLETED; reconcile CLAUDE.md

Native OS drag-out landed (copy-only, prune stays sole deleter).
Milestone 11 complete; M9 slots is the last M-numbered milestone.

PLAN.md

@@ -23,16 +23,6 @@ state persists via the index.
 - [ ] Slot model + slot↔sample assignment.
 - [ ] "Capture to slot N" / "insert slot N" actions, MIDI-bindable.
 
-## Milestone 11 — polish
-**Goal:** Native OS drag-out (remaining polish item). CONTEXT.md Build order 11,
-§Non-goals (drag-out deferred to last).
-**Verify (in DAW):** Drag-out places a valid file in the OS target without
-regressing the precision invariants.
-
-- [ ] Native OS drag-out (deferred final; `InsertMedia` path must already work).
-
----
-
 ## Open questions to resolve during build
 Carried from CONTEXT.md §Open questions — keep visible until each is closed by a
 landed milestone.