feat: enhance Home Assistant integration with service discovery and setup

Improvements to the HA integration merged from PR #184:

- Add ha_list_services tool: discovers available services (actions) per
  domain with descriptions and parameter fields. Tells the model what
  it can do with each device type (e.g. light.turn_on accepts brightness,
  color_name, transition). Closes the gap where the model had to guess
  available actions.

- Add HA to hermes tools config: users can enable/disable the homeassistant
  toolset and configure HASS_TOKEN + HASS_URL through 'hermes tools' setup
  flow instead of manually editing .env.

- Fix should-fix items from code review:
  - Remove sys.path.insert hack from gateway adapter
  - Replace all print() calls with proper logger (info/warning/error)
  - Move env var reads from import-time to handler-time via _get_config()
  - Add dedicated REST session reuse in gateway send()

- Update ha_call_service description to reference ha_list_services for
  action discovery.

- Update tests for new ha_list_services tool in toolset resolution.

tests/gateway/test_homeassistant.py

@@ -569,19 +569,19 @@ class TestToolsetIntegration:
         from toolsets import resolve_toolset
 
         tools = resolve_toolset("homeassistant")
-        assert set(tools) == {"ha_list_entities", "ha_get_state", "ha_call_service"}
+        assert set(tools) == {"ha_list_entities", "ha_get_state", "ha_call_service", "ha_list_services"}
 
     def test_gateway_toolset_includes_ha_tools(self):
         from toolsets import resolve_toolset
 
         gateway_tools = resolve_toolset("hermes-gateway")
-        for tool in ("ha_list_entities", "ha_get_state", "ha_call_service"):
+        for tool in ("ha_list_entities", "ha_get_state", "ha_call_service", "ha_list_services"):
             assert tool in gateway_tools
 
     def test_hermes_core_tools_includes_ha(self):
         from toolsets import _HERMES_CORE_TOOLS
 
-        for tool in ("ha_list_entities", "ha_get_state", "ha_call_service"):
+        for tool in ("ha_list_entities", "ha_get_state", "ha_call_service", "ha_list_services"):
             assert tool in _HERMES_CORE_TOOLS