feat(mcp): add MCP (Model Context Protocol) tool integration

Allows CowAgent to dynamically load tools from any MCP server at startup,
extending the agent from a fixed toolset to an open, extensible tool ecosystem.

## What's added

- `agent/tools/mcp/mcp_client.py`: lightweight JSON-RPC client supporting both
  stdio (subprocess) and SSE (HTTP) transports — zero extra dependencies
- `agent/tools/mcp/mcp_tool.py`: `McpTool` wraps a single MCP tool as a
  `BaseTool`, with dynamic name/description/params set at instance level
- `agent/tools/tool_manager.py`: new `_load_mcp_tools()` loads MCP servers at
  startup via `McpClientRegistry`; falls back gracefully on any error; no-op
  when `mcp_servers` is not configured
- `config.py`: registers `mcp_servers` in `available_setting` with inline docs

## Design

- No new dependencies — JSON-RPC implemented from scratch using stdlib only
- MCP clients are long-lived (initialized once, shared across tool calls)
- `McpClientRegistry` holds all subprocess handles and shuts them down cleanly
- Server init failures are non-fatal: logged as warnings, agent continues normally
- Zero overhead when `mcp_servers` is absent from config

## Config example

```json
"mcp_servers": [
  {
    "name": "filesystem",
    "type": "stdio",
    "command": "npx",
    "args": ["-y", "@modelcontextprotocol/server-filesystem", "/tmp"]
  }
]
```

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent/tools/__init__.py

@@ -107,6 +107,22 @@ def _import_browser_tool():
 
 BrowserTool = _import_browser_tool()
 
+# MCP Tools (no extra dependencies, loaded on demand)
+def _import_mcp_tools():
+    """导入 MCP 工具模块（无额外依赖，按需加载）"""
+    from common.log import logger
+    try:
+        from agent.tools.mcp.mcp_tool import McpTool
+        from agent.tools.mcp.mcp_client import McpClientRegistry
+        return {'McpTool': McpTool, 'McpClientRegistry': McpClientRegistry}
+    except Exception as e:
+        logger.warning(f"[Tools] MCP tools not loaded: {e}")
+        return {}
+
+_mcp_tools = _import_mcp_tools()
+McpTool = _mcp_tools.get('McpTool')
+McpClientRegistry = _mcp_tools.get('McpClientRegistry')
+
 # Export all tools (including optional ones that might be None)
 __all__ = [
     'BaseTool',
@@ -125,6 +141,7 @@ __all__ = [
     'WebFetch',
     'Vision',
     'BrowserTool',
+    'McpTool',
 ]
 
 """