feat(state-hub): CUST-WP-0040 — NATS lifecycle event publishing for activity-core

Makes the state hub an event publisher so activity-core can drive
maintenance automation declaratively via ActivityDefinitions, rather
than the hub creating tasks itself.

- api/events/: lazy JetStream publisher + EventEnvelope mirroring
  activity-core's contract; no-op when NATS_URL unset, fire-and-forget
  with logged failures so publishing never breaks an API request.
- Wired publishers on the five v1.0 lifecycle events:
    org.statehub.repo.registered        (POST /repos/)
    org.statehub.workstream.completed   (PATCH /workstreams/* on transition)
    org.statehub.decision.resolved      (POST /decisions/*/resolve)
    org.statehub.domain.goal.activated  (POST /domain-goals/*/activate)
    org.statehub.task.stale             (scripts/cleanup_stale_tasks.py)
- docs/nats-event-subjects.md: subject naming convention + catalog.
- docs/cron-migration.md: design stub for replacing custodian-sync
  systemd timer and cleanup-stale cron with ActivityDefinitions
  (depends on activity-core WP-0003).
- docs/activity-core-delegation.md: protocol, invariants, cutover plan.
- SCOPE.md: declares activity-core as downstream event consumer and
  restates that the state hub stays a read model, not a task factory.

Workplan: workplans/CUST-WP-0040-state-hub-nats-activity-core-integration.md
242 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

state-hub/api/routers/workstreams.py

@@ -1,3 +1,4 @@
+import asyncio
 import uuid
 import socket
 import time
@@ -9,6 +10,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from api.database import get_session
+from api.events import EventEnvelope, publish_event
 from api.models.managed_repo import ManagedRepo
 from api.models.workstream import Workstream
 from api.schemas.workstream import (
@@ -168,10 +170,27 @@ async def update_workstream(
     ws = await session.get(Workstream, workstream_id)
     if ws is None:
         raise HTTPException(status_code=404, detail="Workstream not found")
+    prev_status = ws.status
     for field, value in body.model_dump(exclude_unset=True).items():
         setattr(ws, field, value)
     await session.commit()
     await session.refresh(ws)
+
+    if prev_status != "completed" and ws.status == "completed":
+        subject = "org.statehub.workstream.completed"
+        envelope = EventEnvelope.new(
+            subject,
+            attributes={
+                "workstream_id": str(ws.id),
+                "slug": ws.slug,
+                "title": ws.title,
+                "topic_id": str(ws.topic_id),
+                "repo_id": str(ws.repo_id) if ws.repo_id else None,
+                "repo_goal_id": str(ws.repo_goal_id) if ws.repo_goal_id else None,
+            },
+        )
+        asyncio.create_task(publish_event(subject, envelope))
+
     return ws