vLLM priority scheduling for agents

Thread request priority through the API call chain to vLLM's
priority scheduler. Lower value = higher priority, with preemption.

Priority is set per-agent in the .agent header:
- interactive (runner): 0 (default, highest)
- surface-observe: 1 (near-realtime, watches conversation)
- all other agents: 10 (batch, default if not specified)

Requires vLLM started with --scheduling-policy priority.

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

src/agent/api/openai.rs

@@ -26,6 +26,7 @@ pub async fn stream_events(
     ui_tx: &UiSender,
     reasoning_effort: &str,
     temperature: Option<f32>,
+    priority: Option<i32>,
 ) -> Result<()> {
     let request = ChatRequest {
         model: model.to_string(),
@@ -44,6 +45,7 @@ pub async fn stream_events(
             None
         },
         chat_template_kwargs: None,
+        priority,
     };
 
     let url = format!("{}/chat/completions", base_url);