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/agent/tools/mod.rs

@@ -118,6 +118,17 @@ pub struct ProcessInfo {
     pub started: Instant,
 }
 
+/// A tool call in flight — metadata for TUI + JoinHandle for
+/// result collection and cancellation.
+pub struct ActiveToolCall {
+    pub id: String,
+    pub name: String,
+    pub detail: String,
+    pub started: Instant,
+    pub background: bool,
+    pub handle: tokio::task::JoinHandle<(ToolCall, ToolOutput)>,
+}
+
 /// Shared tracker for running child processes. Allows the TUI to
 /// display what's running and kill processes by PID.
 #[derive(Debug, Clone, Default)]