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/agent/runner/host_models.py

@@ -0,0 +1,171 @@
+"""Agent event envelope and binding models for LangBot Host.
+
+These are Host-internal models, not exposed to SDK.
+"""
+from __future__ import annotations
+
+import typing
+import pydantic
+
+from langbot_plugin.api.entities.builtin.agent_runner.event import (
+    AgentEventContext,
+    ConversationContext,
+    ActorContext,
+    SubjectContext,
+    RawEventRef,
+)
+from langbot_plugin.api.entities.builtin.agent_runner.input import AgentInput
+from langbot_plugin.api.entities.builtin.agent_runner.delivery import DeliveryContext
+
+
+class AgentEventEnvelope(pydantic.BaseModel):
+    """Event envelope for LangBot Host event gateway.
+
+    This is the unified input model that replaces Query-first approach.
+    IM / WebUI / API / EventRouter all produce this envelope.
+    """
+
+    event_id: str
+    """Unique event identifier."""
+
+    event_type: str
+    """Event type (message.received, message.recalled, etc.)."""
+
+    event_time: int | None = None
+    """Event timestamp (epoch seconds)."""
+
+    source: str
+    """Event source (platform, webui, api, scheduler, system)."""
+
+    bot_id: str | None = None
+    """Bot UUID handling this event."""
+
+    workspace_id: str | None = None
+    """Workspace ID (for multi-tenant)."""
+
+    conversation_id: str | None = None
+    """Conversation ID."""
+
+    thread_id: str | None = None
+    """Thread ID (for platforms supporting threads)."""
+
+    actor: ActorContext | None = None
+    """Actor (who triggered the event)."""
+
+    subject: SubjectContext | None = None
+    """Subject (what the event is about)."""
+
+    input: AgentInput
+    """Event input."""
+
+    delivery: DeliveryContext
+    """Delivery context."""
+
+    raw_ref: RawEventRef | None = None
+    """Reference to raw event payload."""
+
+
+# Binding scope types
+class BindingScope(pydantic.BaseModel):
+    """Scope for agent binding."""
+
+    scope_type: typing.Literal["bot", "pipeline", "workspace", "global"] = "pipeline"
+    """Scope type."""
+
+    scope_id: str | None = None
+    """Scope identifier (bot_uuid, pipeline_uuid, etc.)."""
+
+
+class ResourcePolicy(pydantic.BaseModel):
+    """Resource policy for agent binding.
+
+    Controls what resources the runner can access.
+    """
+
+    allowed_model_uuids: list[str] | None = None
+    """Allowed model UUIDs. None means all authorized."""
+
+    allowed_tool_names: list[str] | None = None
+    """Allowed tool names. None means all authorized."""
+
+    allowed_kb_uuids: list[str] | None = None
+    """Allowed knowledge base UUIDs. None means all authorized."""
+
+    allow_plugin_storage: bool = True
+    """Whether plugin storage is allowed."""
+
+    allow_workspace_storage: bool = False
+    """Whether workspace storage is allowed."""
+
+
+class StatePolicy(pydantic.BaseModel):
+    """State policy for agent binding.
+
+    Controls state management behavior.
+    """
+
+    enable_state: bool = True
+    """Whether host-owned state is enabled."""
+
+    state_scopes: list[typing.Literal["conversation", "actor", "subject", "runner"]] = (
+        pydantic.Field(default_factory=lambda: ["conversation", "actor"])
+    )
+    """Enabled state scopes."""
+
+
+class DeliveryPolicy(pydantic.BaseModel):
+    """Delivery policy for agent binding.
+
+    Controls how results are delivered.
+    """
+
+    enable_streaming: bool = True
+    """Whether streaming output is enabled."""
+
+    enable_reply: bool = True
+    """Whether reply is enabled."""
+
+    max_message_size: int | None = None
+    """Maximum message size."""
+
+
+class AgentBinding(pydantic.BaseModel):
+    """Binding configuration for mapping events to runners.
+
+    This is Host-internal model for event-to-runner binding.
+    It replaces the old Pipeline runner config role.
+    """
+
+    binding_id: str
+    """Unique binding identifier."""
+
+    scope: BindingScope = pydantic.Field(default_factory=BindingScope)
+    """Binding scope."""
+
+    event_types: list[str] = pydantic.Field(default_factory=lambda: ["message.received"])
+    """Event types this binding handles."""
+
+    runner_id: str
+    """Runner ID to invoke."""
+
+    runner_config: dict[str, typing.Any] = pydantic.Field(default_factory=dict)
+    """Runner instance configuration."""
+
+    resource_policy: ResourcePolicy = pydantic.Field(default_factory=ResourcePolicy)
+    """Resource policy."""
+
+    state_policy: StatePolicy = pydantic.Field(default_factory=StatePolicy)
+    """State policy."""
+
+    delivery_policy: DeliveryPolicy = pydantic.Field(default_factory=DeliveryPolicy)
+    """Delivery policy."""
+
+    enabled: bool = True
+    """Whether binding is enabled."""
+
+    # Legacy fields for compatibility adapter
+    pipeline_uuid: str | None = None
+    """Legacy pipeline UUID (for compatibility)."""
+
+    max_round: int | None = None
+    """Legacy max-round (for compatibility adapter, not Protocol v1)."""