Cancel pending timers when a child is closed

Stale timer bodies were re-delivered to the orchestrator pane after
the parent had already processed the sub-agent's reply and called
close_process. The timer registry held no link to the child
lifecycle, so timers owned by or watching the closed child lingered
until something triggered a fire — e.g. a trailing classifier tick
for the now-removed child.

Add an OnChildClosed hook to ChildEventListener, emit it from
Session.Close (and the terminal-corpse path in reapChild), and have
the timer manager prune the registry: cancel timers owned by the
closed child; remove the closed child from each timer's watched
list (cancel the timer outright when watched empties).

Natural exit deliberately does not route through this hook — the
classifier already emits an idle transition on exit which delivers
any legitimate "fire when sub-agent finishes" semantics exactly
once; cancelling on exit would swallow that.

internal/app/app.go

@@ -837,6 +837,13 @@ func (st *uiState) OnChildStateChanged(string, IdleState) {
 	st.drawSidebar()
 }
 
+// OnChildClosed is the explicit-removal hook (close_process or the
+// terminal-corpse cleanup in reapChild). The UI already reflects
+// removals via the OnChildExited path and the children-map view, so
+// this is a no-op here — the timerManager is the consumer that
+// cares.
+func (st *uiState) OnChildClosed(string) {}
+
 // OnChildExited drops focus and shows the empty state if it was the
 // focused child.
 func (st *uiState) OnChildExited(c *Child) {