Feature: allow choosing stable releases vs prereleases for update checks

Summary

Please add a user-configurable update channel for Codex CLI:

• stable (default): check and install only stable releases
• prerelease: allow prereleases (alpha/beta/rc) for both checking and installing

Why

Right now users who want early fixes/features need prerelease support, while others want stable-only behavior.

Expected behavior

• A config setting in config.toml to choose channel.
• The channel affects both:

1. version discovery / prompting (update checks)
2. the actual install command when user selects update

• Default remains stable.

Context

Separate from this PR (Windows updater command execution reliability):

• https://github.com/openai/codex/pull/12465