feat: add MCP (Model Context Protocol) client support

Connect to external MCP servers via stdio transport, discover their tools
at startup, and register them into the hermes-agent tool registry.

- New tools/mcp_tool.py: config loading, server connection via background
  event loop, tool handler factories, discovery, and graceful shutdown
- model_tools.py: trigger MCP discovery after built-in tool imports
- cli.py: call shutdown_mcp_servers in _run_cleanup
- pyproject.toml: add mcp>=1.2.0 as optional dependency
- 27 unit tests covering config, schema conversion, handlers, registration,
  SDK interaction, toolset injection, graceful fallback, and shutdown

Config format (in ~/.hermes/config.yaml):
  mcp_servers:
    filesystem:
      command: "npx"
      args: ["-y", "@modelcontextprotocol/server-filesystem", "/tmp"]

cli.py

@@ -386,6 +386,11 @@ def _run_cleanup():
         _cleanup_all_browsers()
     except Exception:
         pass
+    try:
+        from tools.mcp_tool import shutdown_mcp_servers
+        shutdown_mcp_servers()
+    except Exception:
+        pass
 
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