agent: switch from tokio::sync::Mutex to std::sync::Mutex

The agent lock is never held across await points — turns lock briefly,
do work, drop, then do async API calls. std::sync::Mutex works and
can be locked from sync contexts (screen tick inside terminal.draw).

Fixes: blocking_lock() panic when called inside tokio runtime.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/agent/api/mod.rs

@@ -621,7 +621,7 @@ pub async fn collect_stream(
     rx: &mut mpsc::UnboundedReceiver<StreamEvent>,
     ui_tx: &UiSender,
     target: StreamTarget,
-    agent: &std::sync::Arc<tokio::sync::Mutex<super::Agent>>,
+    agent: &std::sync::Arc<std::sync::Mutex<super::Agent>>,
     active_tools: &crate::user::ui_channel::SharedActiveTools,
 ) -> StreamResult {
     let mut content = String::new();