Fixes#1036
After adding an MCP server to config.yaml, users had to restart Hermes
before the new tools became visible — even though /reload-mcp existed.
Add _check_config_mcp_changes() called from process_loop every 5s:
- stat() config.yaml for mtime changes (fast path, no YAML parse)
- On mtime change, parse and compare mcp_servers section
- If mcp_servers changed, auto-trigger _reload_mcp() and notify user
- Skip check while agent is running to avoid interrupting tool calls
- Throttled to CONFIG_WATCH_INTERVAL=5s to avoid busy-polling
/reload-mcp still works for manual force-reload.
Tests: 6 new tests in TestMCPConfigWatch, all passed
Co-authored-by: teyrebaz33 <hakanerten02@hotmail.com>