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/decisions.py

@@ -1,3 +1,4 @@
+import asyncio
 import logging
 import uuid
 from datetime import datetime, timezone
@@ -10,6 +11,7 @@ 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.decision import Decision, DecisionStatus, DecisionType
 from api.models.progress_event import ProgressEvent
 from api.schemas.decision import DecisionCreate, DecisionRead, DecisionResolve, DecisionUpdate
@@ -150,6 +152,20 @@ async def resolve_decision_action(
     if body.write_log:
         await _write_project_log(decision, body.rationale, body.decided_by, session)
 
+    subject = "org.statehub.decision.resolved"
+    envelope = EventEnvelope.new(
+        subject,
+        attributes={
+            "decision_id": str(decision.id),
+            "title": decision.title,
+            "topic_id": str(decision.topic_id) if decision.topic_id else None,
+            "workstream_id": str(decision.workstream_id) if decision.workstream_id else None,
+            "decided_by": body.decided_by,
+            "rationale_snippet": (body.rationale or "")[:240],
+        },
+    )
+    asyncio.create_task(publish_event(subject, envelope))
+
     return decision