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/mind/mod.rs

@@ -248,7 +248,7 @@ enum BgEvent {
 pub type SharedMindState = std::sync::Mutex<MindState>;
 
 pub struct Mind {
-    pub agent: Arc<tokio::sync::Mutex<Agent>>,
+    pub agent: Arc<Agent>,
     pub shared: Arc<SharedMindState>,
     pub config: SessionConfig,
     subconscious: tokio::sync::Mutex<Subconscious>,