Sprint 2: update runtime __init__ with checkpoint + state exports

purpose_agent/runtime/__init__.py

@@ -3,12 +3,28 @@ Purpose Agent Runtime — Event-driven execution engine for v3.0.
 
 Provides:
   - PAEvent: canonical event schema
-  - EventKind: enumerated event types
+  - EventKind: enumerated event types  
+  - Visibility: event visibility levels
   - EventBus: async pub/sub with backpressure and replay
-  - Checkpointer: durable state snapshots (Sprint 2)
-  - RunState: typed execution state (Sprint 2)
+  - RunState: typed execution state for checkpointing
+  - Checkpointer: protocol for durable state persistence
+  - InMemoryCheckpointer: testing
+  - JSONLCheckpointer: file-based, portable
+  - SQLiteCheckpointer: production, concurrent-safe
 """
-from purpose_agent.runtime.events import PAEvent, EventKind, Visibility
-from purpose_agent.runtime.event_bus import EventBus
+from purpose_agent.runtime.events import PAEvent, EventKind, Visibility, create_event, TERMINAL_KINDS
+from purpose_agent.runtime.event_bus import EventBus, parallel_merge
+from purpose_agent.runtime.state import RunState, RunStatus, NodeState
+from purpose_agent.runtime.checkpoint import (
+    Checkpointer,
+    InMemoryCheckpointer,
+    JSONLCheckpointer, 
+    SQLiteCheckpointer,
+)
 
-__all__ = ["PAEvent", "EventKind", "Visibility", "EventBus"]
+__all__ = [
+    "PAEvent", "EventKind", "Visibility", "create_event", "TERMINAL_KINDS",
+    "EventBus", "parallel_merge",
+    "RunState", "RunStatus", "NodeState",
+    "Checkpointer", "InMemoryCheckpointer", "JSONLCheckpointer", "SQLiteCheckpointer",
+]