WIP: Agent/AgentState — 36 errors remaining, all .lock() → .state.lock() or .context.lock()

Bulk replaced Arc<Mutex<Agent>> with Arc<Agent> across all files.
Fixed control.rs, memory.rs tool handlers. Fixed oneshot Backend.
Remaining errors are all agent.lock() → agent.state.lock() or
agent.context.lock() in mind/, user/, and a few in mod.rs.

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

src/agent/oneshot.rs

@@ -63,11 +63,11 @@ pub struct AutoAgent {
 }
 
 /// Per-run conversation backend — wraps a forked agent.
-struct Backend(std::sync::Arc<tokio::sync::Mutex<Agent>>);
+struct Backend(std::sync::Arc<Agent>);
 
 impl Backend {
     async fn push_node(&mut self, node: AstNode) {
-        self.0.lock().await.push_node(node);
+        self.0.push_node(node).await;
     }
 }
 
@@ -149,7 +149,7 @@ impl AutoAgent {
     /// Arc to read entries live during the run.
     pub async fn run_forked_shared(
         &mut self,
-        agent: &std::sync::Arc<tokio::sync::Mutex<Agent>>,
+        agent: &std::sync::Arc<Agent>,
         memory_keys: &[String],
         state: &std::collections::BTreeMap<String, String>,
     ) -> Result<String, String> {