checkpoint(milestone): v0.4 complete — clear for next milestone

v0.4 Refinement (NFR) milestone complete. Release v0.3.4 (id 752).
All milestone branches deleted (local + remote); tags preserve history.
HEAD on main. NFR purity gate GREEN. Next run starts a new milestone.

---ci---
project: oy
phase: 4
milestone: v0.4
status: complete
tag_base: v0.3.x
milestone_type: nfr
phase_role: final
milestone_complete: true
milestone_release_tag: v0.3.4
release_id: 752
requirements:
  covered: [REQ-029, REQ-030, REQ-031, REQ-032]
  partial: []
---/ci---

.ciagent/CHECKPOINT.json

@@ -1,14 +1,15 @@
 {
-  "phase": 3,
+  "phase": 4,
   "stage": "complete",
   "milestone": "v0.4",
   "milestone_type": "nfr",
   "tag_base": "v0.3.x",
-  "phase_role": "execution",
+  "phase_role": "final",
   "project": "oy",
   "attempts": 0,
-  "updated_at": "2026-08-17T23:45:00Z",
-  "phase_release_tag": "v0.3.3",
-  "release_id": 751,
-  "reqs_covered": ["REQ-029", "REQ-030", "REQ-031", "REQ-032"]
+  "updated_at": "2026-08-17T23:50:00Z",
+  "milestone_complete": true,
+  "milestone_release_tag": "v0.3.4",
+  "release_id": 752,
+  "requirements_covered": ["REQ-029", "REQ-030", "REQ-031", "REQ-032"]
 }