[codex] document submission inspection and deletion flow

[msaroufim]

What changed

Adds a short README note showing the minimal popcorn submissions list/show/delete flow so users can inspect and remove a bad or reward-hacked submission themselves.

Why

Users need an obvious place in the main CLI docs that explains they can:

• list submissions to find the submission ID
• inspect the stored code for that ID
• delete that submission if needed

User impact

People using popcorn-cli can recover from an accidental or reward-hacked submission without asking for manual intervention.

Validation

• Verified the commands against the live service using grayscale_v2
• Confirmed list returned a real submission ID
• Confirmed show printed the stored code for that ID
• Confirmed delete --force removed that submission
• No code changes or automated tests were needed for this README-only update

[msaroufim]

Updated the README framing on this branch. The previous inline note is now a standalone Avoid Reward Hacks section under Submissions, with the same list/show/delete flow but phrased as explicit remediation guidance rather than a side note.