fix(acp): attach error listener to spawned ACP agent process (#109529)

The spawned agent child process in createAcpClient had no error
listener. If the agent binary failed to start, the error event
would be unhandled and could crash the process.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/acp/client.ts

@@ -147,6 +147,10 @@ async function createAcpClient(opts: AcpClientOptions = {}): Promise<AcpClientHa
     throw new Error("Failed to create ACP stdio pipes");
   }
 
+  agent.on("error", (err) => {
+    log(`agent error: ${String(err)}`);
+  });
+
   const input = Writable.toWeb(agent.stdin);
   const output = Readable.toWeb(agent.stdout) as unknown as ReadableStream<Uint8Array>;
   const stream = ndJsonStream(input, output);