feat: for codex exec, if PROMPT is not specified, read from stdin if not a TTY

[bolinfest]

This attempts to make codex exec more flexible in how the prompt can be passed:

• as before, it can be passed as a single string argument
• if - is passed as the value, the prompt is read from stdin
• if no argument is passed and stdin is a tty, prints a warning to stderr that no prompt was specified an exits non-zero.
• if no argument is passed and stdin is NOT a tty, prints Reading prompt from stdin... to stderr to let the user know that Codex will wait until it reads EOF from stdin to proceed. (You can repro this case by doing yes | just exec since stdin is not a TTY in that case but it also never reaches EOF).