fix(tui-v2): detach tool subprocess stdin

[nianyucatfish]

Summary

• Stop code_run tool subprocesses from inheriting the TUI terminal stdin.
• Do the same for tuiapp_v2 !shell commands.
• Keep clipboard copy helpers from inheriting stdin when they are not fed input.

Why

On POSIX terminals, child processes that inherit fd 0 can race Textual for keyboard input while tools are running. This shows up most often on macOS/Linux with multiple active tui2 sessions as randomly swallowed letters, spaces, or Enter.

Notes

• Windows behavior is unchanged.
• Clipboard helpers still receive stdin when input= is provided.
• This intentionally does not change memory/checklist_helper.py from fix(tui v2): stop tool subprocesses inheriting TTY stdin (multi-session key-eating on macOS/Linux) #656.

Verification

• PYTHONIOENCODING=utf-8 py -3.12 -m py_compile ga.py frontends/tuiapp_v2.py
• code_run smoke test: Python child sees stdin EOF (sys.stdin.read() == '') while stdout capture still works.

[lsdefine]

Reviewed the changes and targeted behavior. Approving for merge.