Refactor hook: split agent orchestration from formatting

- Remove POC_AGENT early return (was from old claude -p era)
- Split hook into run_agent_cycles() -> AgentCycleOutput (returns
  memory keys + reflection) and format_agent_output() (renders for
  Claude Code injection). poc-agent can call run_agent_cycles
  directly and handle output its own way.
- Fix UTF-8 panic in runner.rs display_buf slicing (floor_char_boundary)
- Add priority debug label to API requests
- Wire up F2 agents screen: live pid status, output files, hook log
  tail, arrow key navigation, Enter for log detail view

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

src/agent/api/openai.rs

@@ -50,7 +50,11 @@ pub async fn stream_events(
 
     let url = format!("{}/chat/completions", base_url);
     let msg_count = request.messages.len();
-    let debug_label = format!("{} messages, model={}", msg_count, model);
+    let pri_label = match priority {
+        Some(p) => format!(", priority={}", p),
+        None => String::new(),
+    };
+    let debug_label = format!("{} messages, model={}{}", msg_count, model, pri_label);
 
     let mut response = super::send_and_check(
         client,