fix(background-agent): preserve parent activity across idle events

Cubic caught that clearing recent parent activity on session.idle could reopen the stale-idle race. Keep the fresh activity marker until its normal expiry so background completions still defer while the parent turn is actively reasoning.

Red: parent idle after fresh reasoning delta dispatched an unexpected parent wake. Green: focused parent-wake regression suite passes.

src/features/background-agent/manager.ts

@@ -1625,7 +1625,6 @@ The fallback retry session is now created and can be inspected directly.
       if (!props || typeof props !== "object") return
       const sessionID = resolveSessionEventID(props)
       if (sessionID) {
-        this.parentWakeNotifier.clearParentSessionActivity(sessionID)
         void this.enqueueNotificationForParent(sessionID, () => this.flushPendingParentWake(sessionID)).catch((error) => {
           log("[background-agent] Failed to flush pending parent wake:", { sessionID, error })
         })