Revert to tokio::sync::Mutex, fix lock-across-await bugs, move input ownership to InteractScreen

The std::sync::Mutex detour caught every place a MutexGuard lived
across an await point in Agent::turn — the compiler enforced Send
safety that tokio::sync::Mutex silently allows. With those fixed,
switch back to tokio::sync::Mutex (std::sync blocks tokio worker
threads and panics inside the runtime).

Input and command dispatch now live in InteractScreen (chat.rs):
- Enter pushes directly to SharedMindState.input (no app.submitted hop)
- sync_from_agent displays pending input with dimmed color
- Slash command table moved from event_loop.rs to chat.rs
- cmd_switch_model kept as pub fn for tool-initiated switches

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

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<std::sync::Mutex<super::Agent>>,
+    agent: &std::sync::Arc<tokio::sync::Mutex<super::Agent>>,
     active_tools: &crate::user::ui_channel::SharedActiveTools,
 ) -> StreamResult {
     let mut content = String::new();