fix(agent): let shutdown supersede a prior budget abort

requestAbort's abortSent branch only upgraded incoming signal reasons, so a
shutdown landing after a soft-budget hard-abort was discarded and abortKind()
stayed budget. finalizeSubagentLifecycle then followed the budget-resumable
path (idle + adopt) even though AgentLifecycleManager.dispose() had run,
leaking the subagent session into SDK/process reuse.

- Upgrade a prior budget abort to shutdown so the run takes the shutdown
  release path; genuine kills (signal/timeout/terminate) stay terminal and
  shutdown is never downgraded to signal.
- Cover the shutdown-races-budget-hard-abort case.

Fixes #8216

packages/coding-agent/src/task/executor.ts

@@ -1095,7 +1095,21 @@ function createSubagentRunMonitor(args: RunMonitorArgs): SubagentRunMonitor {
 			budgetLimitExceeded = true;
 		}
 		if (abortSent) {
-			if (reason === "signal" && abortReason !== "signal" && abortReason !== "timeout") {
+			// Shutdown is a superseding external abort: a process teardown that
+			// races a self-inflicted budget hard-abort must still follow the
+			// shutdown release path (dispose + unregister) instead of the
+			// budget-resumable path, which would leave the subagent adopted and
+			// alive past AgentLifecycleManager.dispose(). Genuine kills
+			// (signal/timeout/terminate) already dispose terminally, and shutdown
+			// is never downgraded back to signal.
+			if (reason === "shutdown" && abortReason === "budget") {
+				abortReason = "shutdown";
+			} else if (
+				reason === "signal" &&
+				abortReason !== "signal" &&
+				abortReason !== "timeout" &&
+				abortReason !== "shutdown"
+			) {
 				abortReason = "signal";
 			}
 			return;