Feature: allow choosing stable releases vs prereleases for update checks

[mihneaptu]

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):

• fix(cli): run Windows updater via structured cmd invocation #12465

[etraut-openai]

Which variant of Codex are you using? The CLI, IDE extension, or app?

We release the CLI very frequently (sometimes several times a day), so prereleases don't really make sense there. You can install alpha builds if you want.

For the IDE extension, we already release stable and prerelease versions, and you can select between them in VS Code or Cursor.

[mihneaptu]

Got it, thanks for clarifying. I’ll keep using the manual install for now.