feat(agent-runner): add event-first context facts and pull APIs

Add EventLog and Transcript persistence entities for storing auditable
event facts and conversation history projection. Implement event-first
AgentRunContext builder that produces Protocol v1 compliant context
payloads with required fields: event, delivery, context (ContextAccess).

Key changes:
- EventLog ORM: auditable event records with indexes
- Transcript ORM: conversation history projection with composite indexes
- AgentRunContextBuilder: Protocol v1 payload with delivery, context, bootstrap
- EventLogStore/TranscriptStore: async stores for fact sources
- Host action handlers: HISTORY_PAGE, HISTORY_SEARCH, EVENT_GET, EVENT_PAGE
- Context validation: build_context output validates via SDK AgentRunContext
- Alembic migration for event_log and transcript tables
- Alembic env.py imports all ORM models for autogenerate discovery

Legacy compatibility: max-round messages go into bootstrap.messages and
compatibility.legacy_messages, not top-level messages field.

tests/unit_tests/agent/test_context_builder_params_state.py

@@ -420,9 +420,12 @@ class TestBuildParamsInContext:
 
         context = await builder.build_context(query, descriptor, resources)
 
-        assert 'params' in context
-        assert context['params']['public_param'] == 'value'
-        assert '_private' not in context['params']
+        # Protocol v1: params is in compatibility.extra
+        assert 'compatibility' in context
+        assert 'extra' in context['compatibility']
+        assert 'params' in context['compatibility']['extra']
+        assert context['compatibility']['extra']['params']['public_param'] == 'value'
+        assert '_private' not in context['compatibility']['extra']['params']
 
     @pytest.mark.asyncio
     async def test_params_and_state_both_present(self):
@@ -454,10 +457,12 @@ class TestBuildParamsInContext:
 
         context = await builder.build_context(query, descriptor, resources)
 
-        # params should have public vars
-        assert 'params' in context
-        assert context['params']['workflow_input'] == 'user_question'
-        assert context['params']['sender_name'] == 'John'
+        # Protocol v1: params is in compatibility.extra
+        assert 'compatibility' in context
+        assert 'extra' in context['compatibility']
+        assert 'params' in context['compatibility']['extra']
+        assert context['compatibility']['extra']['params']['workflow_input'] == 'user_question'
+        assert context['compatibility']['extra']['params']['sender_name'] == 'John'
 
         # state should have seeded conversation_id
         assert 'state' in context
@@ -490,6 +495,10 @@ class TestBuildParamsInContext:
 
         context = await builder.build_context(query, descriptor, resources)
 
-        assert context['prompt'][0]['content'] == 'Effective prompt'
+        # Protocol v1: prompt is in compatibility.extra
+        assert 'compatibility' in context
+        assert 'extra' in context['compatibility']
+        assert 'prompt' in context['compatibility']['extra']
+        assert context['compatibility']['extra']['prompt'][0]['content'] == 'Effective prompt'
         assert context['runtime']['metadata']['streaming_supported'] is True
         assert context['runtime']['metadata']['remove_think'] is True