Feature request: add `codex update` command for one-step self-update

Problem

Updating Codex CLI depends on install method (npm/brew/binary), so users must remember different commands.

Proposal

Add a built-in update flow:

• codex update --check shows current vs latest version.
• codex update performs update automatically.
• Auto-detect install source and run correct updater (npm, brew, standalone binary).
• Print clear old/new version and actionable failure reasons.

Why this helps

One-step self-update improves discoverability and reduces update friction.

Environment example

Windows + npm global install (@openai/codex).