feat(agent-runner): integrate AgentRunner Protocol v1 with plugin system

Phase 0 integration complete - verified minimal loop with local-agent stub runner.

Changes:
- Add AgentRunOrchestrator for plugin-based agent execution
- Add AgentResultNormalizer for Protocol v1 result conversion
- Add AgentRunnerDescriptor for runner ID parsing (plugin:author/name/runner)
- Update chat handler to use new orchestrator instead of direct runner lookup
- Add plugin handler methods for list_agent_runners and run_agent
- Add connector methods for AgentRunner protocol forwarding
- Update pipeline API to include runner options in metadata
- Add integration docs and implementation plan

Integration verified:
- Runner: plugin:langbot/local-agent/default
- Input: "你好"
- Output: [stub] Echo: 你好
- Date: 2026-05-10 10:09

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/langbot/pkg/core/app.py

@@ -4,6 +4,7 @@ import logging
 import asyncio
 import traceback
 import os
+from typing import TYPE_CHECKING
 
 from ..platform import botmgr as im_mgr
 from ..platform.webhook_pusher import WebhookPusher
@@ -44,6 +45,9 @@ from ..vector import mgr as vectordb_mgr
 from ..telemetry import telemetry as telemetry_module
 from ..survey import manager as survey_module
 
+if TYPE_CHECKING:
+    from ..agent.runner import AgentRunnerRegistry, AgentRunOrchestrator
+
 
 class Application:
     """Runtime application object and context"""
@@ -158,6 +162,11 @@ class Application:
 
     maintenance_service: maintenance_service.MaintenanceService = None
 
+    # Agent runner subsystem
+    agent_runner_registry: AgentRunnerRegistry = None
+
+    agent_run_orchestrator: AgentRunOrchestrator = None
+
     def __init__(self):
         pass