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.

CHANGELOG.md

@@ -33,6 +33,10 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   repaint.
 - Removed the redundant "Back to Settings" row from the
   Agents / Auto-summarization settings screen.
+- Pending `timer_*` entries are now cancelled when their owning or
+  watched child is closed via `close_process`, preventing stale
+  timer bodies from being re-delivered to the orchestrator pane
+  after the work has already been handled.
 
 ## [0.0.6] - 2026-05-15