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/tui/context_screen.rs

@@ -168,7 +168,7 @@ impl App {
         )));
         lines.push(Line::raw(format!("  Reasoning: {}", self.reasoning_effort)));
         lines.push(Line::raw(format!("  Running processes: {}", self.running_processes)));
-        lines.push(Line::raw(format!("  Active tools: {}", self.active_tools.read().unwrap().len())));
+        lines.push(Line::raw(format!("  Active tools: {}", self.active_tools.lock().unwrap().len())));
 
         let block = Block::default()
             .title_top(Line::from(SCREEN_LEGEND).left_aligned())

src/user/tui/main_screen.rs

@@ -17,7 +17,7 @@ impl App {
     /// Draw the main (F1) screen — four-pane layout with status bar.
     pub(crate) fn draw_main(&mut self, frame: &mut Frame, size: Rect) {
         // Main layout: content area + active tools overlay + status bar
-        let active_tools = self.active_tools.read().unwrap();
+        let active_tools = self.active_tools.lock().unwrap();
         let tool_lines = active_tools.len() as u16;
         let main_chunks = Layout::default()
             .direction(Direction::Vertical)

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.