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/domain_goals.py

@@ -1,3 +1,4 @@
+import asyncio
 import uuid
 
 from fastapi import APIRouter, Depends, HTTPException, status
@@ -5,6 +6,7 @@ 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.domain import Domain
 from api.models.domain_goal import DomainGoal, DomainGoalStatus  # noqa: F401 (DomainGoalStatus used in activate)
 from api.schemas.domain_goal import DomainGoalCreate, DomainGoalRead, DomainGoalUpdate
@@ -96,6 +98,7 @@ async def activate_domain_goal(
     goal = await session.get(DomainGoal, goal_id)
     if goal is None:
         raise HTTPException(status_code=404, detail="Domain goal not found")
+    was_active = goal.status == DomainGoalStatus.active
 
     # Supersede any other active goal for this domain
     existing = await session.execute(
@@ -105,10 +108,28 @@ async def activate_domain_goal(
             DomainGoal.id != goal_id,
         )
     )
+    superseded_ids: list[str] = []
     for old in existing.scalars().all():
         old.status = DomainGoalStatus.superseded
+        superseded_ids.append(str(old.id))
 
     goal.status = DomainGoalStatus.active
     await session.commit()
     await session.refresh(goal)
+
+    if not was_active:
+        domain = await session.get(Domain, goal.domain_id)
+        subject = "org.statehub.domain.goal.activated"
+        envelope = EventEnvelope.new(
+            subject,
+            attributes={
+                "goal_id": str(goal.id),
+                "domain_id": str(goal.domain_id),
+                "domain_slug": domain.slug if domain else None,
+                "title": goal.title,
+                "superseded_goal_ids": superseded_ids,
+            },
+        )
+        asyncio.create_task(publish_event(subject, envelope))
+
     return goal