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/agent/__init__.py

@@ -0,0 +1,37 @@
+"""Agent runner subsystem for LangBot."""
+from __future__ import annotations
+
+from .runner.descriptor import AgentRunnerDescriptor
+from .runner.id import parse_runner_id, format_runner_id, RunnerIdParts, is_plugin_runner_id
+from .runner.errors import (
+    AgentRunnerError,
+    RunnerNotFoundError,
+    RunnerNotAuthorizedError,
+    RunnerProtocolError,
+    RunnerExecutionError,
+)
+from .runner.registry import AgentRunnerRegistry
+from .runner.context_builder import AgentRunContextBuilder
+from .runner.resource_builder import AgentResourceBuilder
+from .runner.result_normalizer import AgentResultNormalizer
+from .runner.orchestrator import AgentRunOrchestrator
+from .runner.config_migration import ConfigMigration
+
+__all__ = [
+    'AgentRunnerDescriptor',
+    'parse_runner_id',
+    'format_runner_id',
+    'is_plugin_runner_id',
+    'RunnerIdParts',
+    'AgentRunnerError',
+    'RunnerNotFoundError',
+    'RunnerNotAuthorizedError',
+    'RunnerProtocolError',
+    'RunnerExecutionError',
+    'AgentRunnerRegistry',
+    'AgentRunContextBuilder',
+    'AgentResourceBuilder',
+    'AgentResultNormalizer',
+    'AgentRunOrchestrator',
+    'ConfigMigration',
+]