feat: render TUI on /dev/tty for Helix and other editor embedding

[leno23]

Summary

Fixes #2556.

Helix :insert-output captures stdout, so gitui could not open its TUI. This change:

• On Unix, if stdout is not a terminal, automatically opens /dev/tty for rendering (covers Helix without gitui >/dev/tty in the keymap).
• Adds --tty to force /dev/tty on Unix (documented in --help).
• Routes terminal setup, shutdown, external editor screen switching, and ratatui through a shared TerminalWriter.

Windows: --tty returns an error; normal stdout behavior is unchanged.

Helix usage

[keys.normal]
C-g = [
  ":write-all",
  ":new",
  ":insert-output gitui",
  ":buffer-close!",
  ":redraw",
  ":reload-all",
]

Test plan

• cargo check
• cargo test verify_app
• CI

Made with Cursor

[leno23]

Pushed a follow-up: --tty now returns an error if /dev/tty cannot be opened (helps macOS permission cases from the issue thread). Local make clippy and cargo test terminal_io pass.

Note: the CI workflow shows action_required for this fork PR — a maintainer may need to approve running workflows on the first contribution from this fork.