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.

src/langbot/pkg/persistence/alembic/env.py

@@ -13,6 +13,26 @@ from sqlalchemy.engine import Connection
 
 from langbot.pkg.entity.persistence.base import Base
 
+# Import all ORM models so they are registered with Base.metadata
+# This is required for autogenerate to detect model changes
+from langbot.pkg.entity.persistence import (
+    apikey,
+    bot,
+    bstorage,
+    event_log,
+    mcp,
+    metadata,
+    model,
+    monitoring,
+    pipeline,
+    plugin,
+    rag,
+    transcript,
+    user,
+    vector,
+    webhook,
+)
+
 target_metadata = Base.metadata