Support dumb terminal mode (TERM=dumb)

I'd like to use Copilot CLI from within Acme (http://acme.cat-v.org/), the Plan 9-style editor, whose _win_ command provides a dumb terminal — no ANSI escape codes, no cursor movement sequences.

Currently the CLI renders a full TUI that produces garbled output in dumb terminals. It would be helpful if the CLI detected `TERM=dumb` and fell back to a simple line-oriented interaction mode — plain prompts, plain responses, no escape codes.

This is distinct from the existing -p prompt flag, which takes a single prompt and exits; a dumb terminal mode would support full multi-turn interactive sessions over a line-oriented interface.

This would also benefit other dumb-terminal environments (e.g. Emacs M-x shell, script pipes, etc.).

Expected behaviour: TERM=dumb copilot starts a plain line-oriented session.
Actual behaviour: The TUI renders escape codes producing garbled output, and input is unusable — the CLI expects cursor movement keys rather than accepting plain text input.