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/runner/descriptor.py

@@ -0,0 +1,72 @@
+"""Agent runner descriptor."""
+from __future__ import annotations
+
+import typing
+import pydantic
+
+
+class AgentRunnerDescriptor(pydantic.BaseModel):
+    """Descriptor for an agent runner.
+
+    Represents the discovered metadata for a runner, including
+    its identity, capabilities, permissions, and configuration schema.
+    """
+
+    id: str
+    """Unique runner ID: plugin:author/plugin_name/runner_name"""
+
+    source: typing.Literal['plugin']
+    """Runner source type"""
+
+    label: dict[str, str]
+    """Display labels keyed by locale (e.g., en_US, zh_Hans)"""
+
+    description: dict[str, str] | None = None
+    """Optional description keyed by locale"""
+
+    plugin_author: str
+    """Plugin author from manifest"""
+
+    plugin_name: str
+    """Plugin name from manifest"""
+
+    runner_name: str
+    """AgentRunner component name from manifest"""
+
+    plugin_version: str | None = None
+    """Optional plugin version"""
+
+    protocol_version: str = '1'
+    """SDK protocol version, default '1'"""
+
+    config_schema: list[dict[str, typing.Any]] = []
+    """Configuration schema using DynamicForm format"""
+
+    capabilities: dict[str, bool] = {}
+    """Runner capabilities: streaming, tool_calling, knowledge_retrieval, etc."""
+
+    permissions: dict[str, list[str]] = {}
+    """Requested permissions: models, tools, knowledge_bases, storage, files, platform_api"""
+
+    raw_manifest: dict[str, typing.Any] = {}
+    """Original manifest for reference"""
+
+    model_config = pydantic.ConfigDict(
+        extra='allow',
+    )
+
+    def get_plugin_id(self) -> str:
+        """Return plugin identifier as author/name."""
+        return f'{self.plugin_author}/{self.plugin_name}'
+
+    def supports_streaming(self) -> bool:
+        """Check if runner supports streaming output."""
+        return self.capabilities.get('streaming', False)
+
+    def supports_tool_calling(self) -> bool:
+        """Check if runner supports tool calling."""
+        return self.capabilities.get('tool_calling', False)
+
+    def supports_knowledge_retrieval(self) -> bool:
+        """Check if runner supports knowledge retrieval."""
+        return self.capabilities.get('knowledge_retrieval', False)