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"]

pyproject.toml

@@ -47,6 +47,7 @@ cli = ["simple-term-menu"]
 tts-premium = ["elevenlabs"]
 pty = ["ptyprocess>=0.7.0"]
 honcho = ["honcho-ai>=2.0.1"]
+mcp = ["mcp>=1.2.0"]
 all = [
   "hermes-agent[modal]",
   "hermes-agent[messaging]",
@@ -57,6 +58,7 @@ all = [
   "hermes-agent[slack]",
   "hermes-agent[pty]",
   "hermes-agent[honcho]",
+  "hermes-agent[mcp]",
 ]
 
 [project.scripts]