ActiveTools wrapper: replace SharedActiveTools Arc<Mutex<Vec>>

New ActiveTools struct with proper methods: push, remove, abort_all,
take_finished, take_foreground, iter, len. Lives directly on AgentState,
no separate Arc<Mutex> needed.

TUI reads active tools through agent.state.try_lock(). Turn loop uses
helpers instead of manual index iteration.

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

src/agent/tools/mod.rs

@@ -21,7 +21,6 @@ mod vision;
 
 use std::future::Future;
 use std::pin::Pin;
-use std::sync::Arc;
 use std::time::Instant;
 
 fn default_timeout() -> u64 { 120 }
@@ -108,6 +107,12 @@ impl ActiveTools {
         self.0.iter()
     }
 
+    pub fn abort_all(&mut self) {
+        for entry in self.0.drain(..) {
+            entry.handle.abort();
+        }
+    }
+
     pub fn len(&self) -> usize { self.0.len() }
     pub fn is_empty(&self) -> bool { self.0.is_empty() }
 }