feat: Add Codex completions

[PRO-2684]

Implemented using Codex itself. Verified with toolkit check pr and completion check:

Target Codex version: codex-cli 0.148.0.

[fdncred]

Why are there only a few with comments/help strings in the screenshot and the code?

[PRO-2684]

Why are there only a few with comments/help strings in the screenshot and the code?

Comments are copied manually to most of the top-level commands, but yes, some are missing. I've built Codex with clap_complete_nushell, since it doesn't support NuShell natively (openai/codex#6030), and generated a decent completion script. This version looks far more better, with help message for most command and flag:

Screenshots

Still some caveats though:

• Recursive help externs (codex exec help, codex exec help resume, codex exec help fork, codex exec help help, codex login help help...)
• Some aliases are missing, like a and e.

[fdncred]

Looks better. Thanks!