feat(event-bridge): WP-0003a — domain model, rules module, event type registry

Implements phases 7–8 of the Event Bridge architecture (custodian-WP-0003a).

Domain model (T34, T40):
- Added RuleDef, InstructionDef, ActionDef to models.py
- Updated ActivityDefinition with rules/instructions fields (task_templates deprecated)
- Formalized EventEnvelope: id, type, version, timestamp, publisher, attributes
- Added from_nats_message() and from_webhook_payload() classmethods

Rules module (T35, T36, T37):
- src/activity_core/rules/ skeleton with boundary enforcement
- evaluate_condition() — sandboxed AST walker, whitelisted nodes only, never exec()
- execute_instruction() — LLM task generation with trusted_fields injection guard
- tests/rules/test_boundary.py verifies no cross-boundary imports

Infrastructure (T38, T39):
- Alembic migrations 0004 (task_spawn_log) and 0005 (event_types)
- IssueSink ABC + IssueCoreRestSink (REST) + NullSink (testing)
- TaskSpawnLog and EventType ORM models

Event type registry (T41, T42, T43):
- event_type_registry.py: file scanner, parser, DB sync, in-process lookup
- ACTIVITY_CURATOR_GATE env var (disabled|required) + approve endpoint
- Three org event type definitions: org.repo.registered, org.workstream.completed,
  org.activity.run.completed

All 10 tests pass. Boundary test confirms rules/ isolation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -11,6 +11,7 @@ dependencies = [
     "uvicorn[standard]>=0.32",
     "alembic>=1.14",
     "nats-py>=2.7",
+    "httpx>=0.27",
 ]
 
 [project.optional-dependencies]