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.

Start by tracing CLI startup into the TUI initialization and its input/output handling; the issue does not name specific files or tests. Verify TERM=dumb selects a multi-turn line-oriented session with plain prompts and responses, no escape codes, and usable plain-text input, while keeping the existing -p behavior distinct.

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