Wire vLLM priority scheduling through all agent paths

The priority field existed in agent definitions and was serialized
into vLLM requests, but was never actually set — every request went
out with no priority, so vLLM treated them equally. This meant
background graph maintenance agents could preempt the main
conversation.

Add priority to AgentState and set it at each call site:
  0 = interactive (main conversation)
  1 = surface agent (needs to feed memories promptly)
  2 = other subconscious agents
  10 = unconscious/standalone agents (batch)

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/oneshot.rs

@@ -143,6 +143,7 @@ impl AutoAgent {
             let mut st = agent.state.lock().await;
             st.provenance = format!("standalone:{}", self.name);
             st.tools = self.tools.clone();
+            st.priority = Some(10);
         }
 
         let mut backend = Backend(agent);