non-interactive support for fork

What feature would you like to see?

Support non-interactive fork, similar to /fork.

Consider the following scenario:

codex exec "Create hello world example in hello.py"
# Now we are at a checkpoint, so we fork.
codex exec fork --last

codex exec "Create a similar program in C."  resume --last 
codex exec "Create a similar program in Javascript."  resume --last

Now we decide that the additional programs were not needed, We re-fork.

codex exec resume --parent
codex exec fork --last

Or we could just re-fork with:

codex exec fork --parent

The changes implied in above:

• Add support for 'exec fork', which creates a new derrived session and exits.
• exec fork could allow --last, --parent or a specific session id.

Optional changes would be similar for exec resume:

• Allow codex exec resume to work without prompt. Without a prompt, it would just reset the current session (if needed) and exit.
• Allow --parent to be passed to exec resume.

The above allows forking at some checkpoint, and going back to that checkpoint non-interactivly, without determining session id's.

As an alternative to using --parent, codex could expose /status as 'codex exec status'. This could allow:

parent=$(codex exec status | grep 'Forked from:' | sed 's/.: //')"

Additional information

_No response_