refactor: remove legacy assistant compatibility

backend/services/brains/prompt_brain.py

@@ -364,31 +364,14 @@ class PromptBrain(BaseBrain):
         )
 
     def _opening_message(self) -> dict[str, Any] | None:
-        startup = self._cfg.startup if isinstance(self._cfg.startup, dict) else {}
-        value = startup.get("opening_message", startup.get("openingMessage"))
+        value = self._cfg.startup["opening_message"]
         return value if isinstance(value, dict) else None
 
     def _opening_mode(self) -> str:
-        startup = self._cfg.startup if isinstance(self._cfg.startup, dict) else {}
-        value = str(startup.get("opening_mode", startup.get("openingMode")) or "")
-        if value in {"interruptible", "playback", "confirmation"}:
-            return value
-        # Before openingMode existed, an opening message meant confirmation;
-        # ordinary Prompt greetings were interruptible.
-        return (
-            "confirmation"
-            if self._opening_message() is not None
-            else "interruptible"
-        )
+        return str(self._cfg.startup["opening_mode"])
 
     def _entry_mode(self) -> str:
-        startup = self._cfg.startup if isinstance(self._cfg.startup, dict) else {}
-        value = str(startup.get("entry_mode", startup.get("entryMode")) or "")
-        if value in {"wait_user", "generate"}:
-            return value
-        # Raw runtime configs saved before entryMode existed generated a first
-        # reply after their opening confirmation. Keep that legacy behavior.
-        return "generate" if self._opening_mode() == "confirmation" else "wait_user"
+        return str(self._cfg.startup["entry_mode"])
 
     async def _enter_prompt_if_ready(self) -> None:
         if (