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/repos.py

@@ -16,6 +16,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from api.config import settings
 from api.database import get_session
+from api.events import EventEnvelope, publish_event
 from api.doi_engine import (
     compute_fingerprint,
     evaluate as _doi_evaluate,
@@ -94,6 +95,20 @@ async def register_repo(
     session.add(repo)
     await session.commit()
     await session.refresh(repo)
+
+    subject = "org.statehub.repo.registered"
+    envelope = EventEnvelope.new(
+        subject,
+        attributes={
+            "repo_id": str(repo.id),
+            "repo_slug": repo.slug,
+            "domain_slug": body.domain_slug,
+            "remote_url": repo.remote_url,
+            "local_path": repo.local_path,
+        },
+    )
+    asyncio.create_task(publish_event(subject, envelope))
+
     return repo