TUI input can be consumed by tool subprocesses #41612

Description

The TUI and tool runtime share file descriptor 0. A custom tool or plugin that starts a subprocess with inherited stdin can consume keystrokes intended for the TUI, leaving the interface unresponsive or corrupted. Shell tool commands already ignore stdin, but other tool execution paths can still inherit it.

Plugins

Reproduces with a local custom tool that spawns a command with inherited stdin.

Steps to reproduce

1. Add a custom tool that starts a subprocess which reads inherited stdin.
2. Invoke the tool from the TUI.
3. Type while the subprocess is waiting for input.
4. The subprocess receives the keystrokes instead of the TUI.

Operating System

macOS and Linux; the same descriptor sharing applies on Windows.

Terminal

Any interactive terminal.