Add codex update command (#19933)

## 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`

codex-rs/cli/tests/marketplace_upgrade.rs

@@ -30,7 +30,7 @@ async fn marketplace_upgrade_no_longer_runs_at_top_level() -> Result<()> {
         .args(["marketplace", "upgrade"])
         .assert()
         .failure()
-        .stderr(contains("unexpected argument 'upgrade' found"));
+        .stderr(contains("unrecognized subcommand 'upgrade'"));
 
     Ok(())
 }