feat: schedule Phase 5 checks; start T06 path decoupling

Add phase5 daily/closeout ActivityDefinitions, document schedules in Phase 5
evidence, and migrate daily triage sinks to custodian:// runtime URIs.

activity-definitions/phase5-stabilization-closeout.md

@@ -0,0 +1,55 @@
+---
+id: "e7d2b5a8-4c1f-4e9a-b6d3-8f2a1c4e6b09"
+name: "Phase 5 Stabilization Closeout Check"
+type: activity-definition
+version: "1.0"
+enabled: true
+owner: custodian
+governance: custodian
+status: active
+created: "2026-07-06"
+trigger:
+  type: scheduled
+  at: "2026-07-09T17:35:00+00:00"
+  timezone: UTC
+context_sources:
+  - type: state-hub
+    query: phase5_stabilization_check
+    required: true
+    params:
+      source: activity-core
+      closeout: true
+      window_start: "2026-07-06T17:35:00+00:00"
+      window_end: "2026-07-09T17:35:00+00:00"
+      baseline:
+        workstreams: 640
+        tasks: 4002
+        topics: 14
+      sweep_limit: 6
+      triage_max_age_hours: 36
+      evidence_sinks:
+        - type: state-hub-progress
+          event_type: phase5_stabilization_closeout
+          author: activity-core
+          workstream_id: 8a828444-dd49-4d7b-a2d1-9952b5bc929d
+          task_id: e91db8d0-973d-4a31-b3c2-ca37fd002ec7
+    bind_to: context.phase5_stabilization_check
+---
+
+# ActivityDefinition: Phase 5 Stabilization Closeout Check
+
+## Purpose
+
+One-shot closeout gate at the end of the `CUST-WP-0054-T05` stabilization
+window. Fails visibly when hub-visible gates are not green. When gates pass,
+records `operator_signoff_needed: true` for coulombcore teardown approval.
+
+## Trigger
+
+Scheduled once at **2026-07-09T17:35:00Z** (72h after window open).
+
+## Output Contract
+
+- one State Hub progress event with `event_type: phase5_stabilization_closeout`
+- fails the ActivityRun when any gate is red at closeout time
+- no LLM call and no direct workplan edits