Add branch override for cloud exec git reference

[squinlan-oai]

Summary

• add a shared git-ref resolver and use it for codex cloud exec and TUI task submission
• expose a new --branch flag to override the git ref passed to cloud tasks
• cover the git-ref resolution behavior with new async unit tests and supporting dev dependencies

Testing

• cargo test -p codex-cloud-tasks

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Codex Task

[github-actions[bot]]

All contributors have signed the CLA ✍️ ✅
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[squinlan-oai]

I have read the CLA Document and I hereby sign the CLA

[squinlan-oai]

@tamird github thought you were relevant.

[tamird]

It thought wrong!

[etraut-openai]

@squinlan-oai, PR has been reviewed and approved. Please address the merge conflicts, and then it should be ready.