unify tool tracking: ActiveToolCall with JoinHandle

One data structure for all in-flight tool calls — metadata for
TUI display + JoinHandle for result collection and cancellation.
Agent spawns tool calls via tokio::spawn, pushes to shared
Arc<Mutex<Vec<ActiveToolCall>>>. TUI reads metadata, can abort().
No separate inflight/background collections.

Non-background: awaited after stream ends.
Background: persists, drained at next turn start.

Co-Developed-By: Kent Overstreet <kent.overstreet@linux.dev>

src/user/ui_channel.rs

@@ -22,20 +22,14 @@ pub fn shared_context_state() -> SharedContextState {
     Arc::new(RwLock::new(Vec::new()))
 }
 
-/// Active tool info for TUI display.
-#[derive(Debug, Clone)]
-pub struct ActiveTool {
-    pub id: String,
-    pub name: String,
-    pub detail: String,
-    pub started: std::time::Instant,
-}
+// ActiveToolCall lives in agent::tools — re-export for TUI access
+pub use crate::agent::tools::ActiveToolCall;
 
-/// Shared active tools — agent writes, TUI reads.
-pub type SharedActiveTools = Arc<RwLock<Vec<ActiveTool>>>;
+/// Shared active tool calls — agent spawns, TUI reads metadata / aborts.
+pub type SharedActiveTools = Arc<std::sync::Mutex<Vec<ActiveToolCall>>>;
 
 pub fn shared_active_tools() -> SharedActiveTools {
-    Arc::new(RwLock::new(Vec::new()))
+    Arc::new(std::sync::Mutex::new(Vec::new()))
 }
 
 /// Which pane streaming text should go to.