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"]

model_tools.py

@@ -105,6 +105,13 @@ def _discover_tools():
 
 _discover_tools()
 
+# MCP tool discovery (external MCP servers from config)
+try:
+    from tools.mcp_tool import discover_mcp_tools
+    discover_mcp_tools()
+except Exception as e:
+    logger.debug("MCP tool discovery failed: %s", e)
+
 
 # =============================================================================
 # Backward-compat constants  (built once after discovery)