feat: add prompt startup actions and shared vision config

backend/services/brains/base.py

@@ -118,6 +118,9 @@ class BaseBrain:
     async def setup(self, cfg: AssistantConfig, runtime: BrainRuntime) -> None:
         """Register tools and initialize per-call orchestration."""
 
+    async def run_preflight(self) -> None:
+        """Run deterministic server-side startup work before media starts."""
+
     async def on_connected(self, *, greeting_pending: bool = False) -> None:
         """Handle a connected client before an optional greeting is played.
 
@@ -198,6 +201,8 @@ class Brain(Protocol):
 
     async def setup(self, cfg: AssistantConfig, runtime: BrainRuntime) -> None: ...
 
+    async def run_preflight(self) -> None: ...
+
     async def on_connected(self, *, greeting_pending: bool = False) -> None: ...
 
     async def on_greeting_finished(self) -> None: ...