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/entity.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from homeassistant.core import callback
 from homeassistant.helpers.dispatcher import async_dispatcher_connect
 from homeassistant.helpers.entity import DeviceInfo, Entity
 
@@ -35,5 +36,9 @@ class ReticulumEntity(Entity):
             )
         )
 
+    @callback
     def _handle_update(self) -> None:
+        # Decorated with @callback so the dispatcher runs it on the event loop
+        # (a plain sync callback would be offloaded to an executor thread, and
+        # async_write_ha_state is not thread-safe).
         self.async_write_ha_state()