fix(tui): Fail when stdin is not a terminal

[joshka-oai]

Piping to codex fails to do anything useful and locks up the process.
We currently check for stdout, but not stdin

❯ echo foo|just c
cargo run --bin codex -- "$@"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.21s
     Running `target/debug/codex`
Error: stdin is not a terminal
error: Recipe `codex` failed on line 10 with exit code 1