feat: add --promote-alpha option to create_github_release script

[bolinfest]

Historically, running create_github_release --publish-release would always publish a new release from latest main, which isn't always the best idea. We should really publish an alpha, let it bake, and then promote it.

This PR introduces a new flag, --promote-alpha, which does exactly that. It also works with --dry-run, so you can sanity check the commit it will use as the base commit for the new release before running it for real.

$ ./codex-rs/scripts/create_github_release --dry-run --promote-alpha 0.56.0-alpha.2
Publishing version 0.56.0
Running gh api GET /repos/openai/codex/git/refs/tags/rust-v0.56.0-alpha.2
Running gh api GET /repos/openai/codex/git/tags/7d4ef77bc35b011aa0c76c5cbe6cd7d3e53f1dfe
Running gh api GET /repos/openai/codex/compare/main...8b49211e67d3c863df5ecc13fc5f88516a20fa69
Would publish version 0.56.0 using base commit 62474a30e8b4a69a8639ae0889bcf7f576dc82f7 derived from rust-v0.56.0-alpha.2.