Fix thread-unsafe state writes; use conversation-agent dropdown

- entity.py: decorate the dispatcher target _handle_update with @callback so it
  runs on the event loop. Without it, async_dispatcher_send offloaded the plain
  sync callback to an executor thread, and async_write_ha_state raised
  "calls async_write_ha_state from a thread other than the event loop".
- config_flow.py: replace the free-text Assist agent field with
  ConversationAgentSelector (a proper agent dropdown), and use LanguageSelector
  for the language field.
- README: clarify that the built-in Assist agent only handles device commands;
  an LLM conversation agent is required for free-form chat.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

custom_components/reticulum/config_flow.py

@@ -17,6 +17,10 @@ from homeassistant.core import callback
 from homeassistant.helpers import config_validation as cv
 from homeassistant.helpers.selector import (
     BooleanSelector,
+    ConversationAgentSelector,
+    ConversationAgentSelectorConfig,
+    LanguageSelector,
+    LanguageSelectorConfig,
     NumberSelector,
     NumberSelectorConfig,
     NumberSelectorMode,
@@ -170,16 +174,14 @@ class ReticulumOptionsFlow(OptionsFlow):
                 ): BooleanSelector(),
                 vol.Optional(
                     CONF_ASSIST_AGENT,
-                    description={
-                        "suggested_value": opts.get(CONF_ASSIST_AGENT, "")
-                    },
-                ): TextSelector(),
+                    description={"suggested_value": opts.get(CONF_ASSIST_AGENT)},
+                ): ConversationAgentSelector(ConversationAgentSelectorConfig()),
                 vol.Optional(
                     CONF_ASSIST_LANGUAGE,
                     description={
-                        "suggested_value": opts.get(CONF_ASSIST_LANGUAGE, "")
+                        "suggested_value": opts.get(CONF_ASSIST_LANGUAGE)
                     },
-                ): TextSelector(),
+                ): LanguageSelector(LanguageSelectorConfig()),
                 vol.Required(
                     CONF_ALLOW_ALL, default=opts.get(CONF_ALLOW_ALL, True)
                 ): BooleanSelector(),