Add codex update command

[etraut-openai]

Why

Addresses #9274

Running codex update currently starts an interactive Codex session with update as the prompt. That is a rough edge for users who expect a direct self-update command after seeing the existing update notice, and it forces them to copy the suggested package-manager command manually.

What changed

• Added a top-level codex update subcommand.
• Reused the existing install-channel detection and update command runner that the TUI already uses for update prompts.
• Exposed the update-action lookup from codex-tui so the CLI can invoke the same behavior.
• Added CLI coverage to ensure codex update is parsed as a subcommand instead of becoming an interactive prompt.

Verification

• cargo test -p codex-cli
• cargo test -p codex-tui update_action::tests