fix(interactive-subagents): harden parallel lifecycle delivery

extensions/interactive-subagents/pi-extension/subagents/orca.ts

@@ -281,11 +281,13 @@ async function readScreenWithCli(
   surface: string,
   lines: number,
   timeout = ASYNC_READ_SCREEN_TIMEOUT_MS,
+  signal?: AbortSignal,
 ): Promise<string> {
   const { stdout } = await execFileAsync(cli, readScreenArgs(surface, lines), {
     encoding: "utf8",
     timeout,
     killSignal: "SIGKILL",
+    signal,
   });
   return parseReadTail(stdout);
 }
@@ -294,6 +296,15 @@ export async function readScreenAsync(surface: string, lines = 50): Promise<stri
   return readScreenWithCli(requireOrca(), surface, lines);
 }
 
+export async function isSurfaceAlive(surface: string, signal?: AbortSignal): Promise<boolean> {
+  try {
+    await readScreenWithCli(requireOrca(), surface, 1, ASYNC_READ_SCREEN_TIMEOUT_MS, signal);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 export const __orcaTest__ = { readScreenWithCli };
 
 export function closeSurface(surface: string): void {
@@ -301,7 +312,11 @@ export function closeSurface(surface: string): void {
   const args = ["terminal", "close", "--terminal", surface];
   if (tabByHandle.has(surface)) args.push("--tab");
   args.push("--json");
-  const output = execFileSync(cli, args, { encoding: "utf8" });
-  parseResponse("terminal close", output);
+  try {
+    const output = execFileSync(cli, args, { encoding: "utf8" });
+    parseResponse("terminal close", output);
+  } catch {
+    // Replaced-shell launches may close their own terminal before cleanup.
+  }
   tabByHandle.delete(surface);
 }