fix(core): keep spawned process stdin open

- Keep public spawned-process stdin open until the caller closes it.
- Preserve the public `writeStdin` contract used by Dynamic Apps and other streaming callers.

Author: NathanFlurry

packages/core/src/agent-os.ts

@@ -4596,6 +4596,7 @@ export class AgentOs {
 			env: options.env,
 			stdin: options.stdin,
 			timeout: options.timeoutMs,
+			streamStdin: true,
 			onStdout: (data) => {
 				recordOutput("stdout", data);
 				options?.onStdout?.(data);