Cancel pending timers when a child is closed (#6)

Co-authored-by: Harry Bayliss <harry@hjb.dev>
Co-committed-by: Harry Bayliss <harry@hjb.dev>

internal/app/debug.go

@@ -111,6 +111,13 @@ func (d *debugCapture) OnChildStateChanged(id string, state IdleState) {
 	})
 }
 
+func (d *debugCapture) OnChildClosed(id string) {
+	d.writeEvent("child_closed", map[string]any{
+		"time": time.Now().Format(time.RFC3339Nano),
+		"id":   id,
+	})
+}
+
 func (d *debugCapture) OnPTYOut(childID string, chunk []byte) {
 	if len(chunk) == 0 {
 		return