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/tools/mod.rs

@@ -29,7 +29,7 @@ fn default_timeout() -> u64 { 120 }
 /// Async tool handler function.
 /// Agent is None when called from contexts without an agent (MCP server, subconscious).
 pub type ToolHandler = fn(
-    Option<std::sync::Arc<std::sync::Mutex<super::Agent>>>,
+    Option<std::sync::Arc<tokio::sync::Mutex<super::Agent>>>,
     serde_json::Value,
 ) -> Pin<Box<dyn Future<Output = anyhow::Result<String>> + Send>>;
 
@@ -94,11 +94,11 @@ pub async fn dispatch(
 pub async fn dispatch_with_agent(
     name: &str,
     args: &serde_json::Value,
-    agent: Option<std::sync::Arc<std::sync::Mutex<super::Agent>>>,
+    agent: Option<std::sync::Arc<tokio::sync::Mutex<super::Agent>>>,
 ) -> String {
     // Look up in agent's tools if available, otherwise global
     let tool = if let Some(ref a) = agent {
-        let guard = a.lock().unwrap();
+        let guard = a.lock().await;
         guard.tools.iter().find(|t| t.name == name).copied()
     } else {
         None