fix(cli): setting a project up for Codex writes a model the account cannot use

[blafourcade]

Setting a project up for Codex writes a model name that a ChatGPT-account Codex refuses. Every session in that project fails until someone edits the file by hand.

What was measured

aidd setup --ai codex --source local --path <repo> --plugins none --yes

writes into the project's .codex/config.toml:

model = "gpt-5"
approval_policy = "on-request"

and the next session ends before it starts:

ERROR: {"type":"error","status":400,"error":{"type":"invalid_request_error",
 "message":"The 'gpt-5' model is not supported when using Codex with a ChatGPT account."}}

Twice, on two fresh projects. It resolves by overriding the model, either in the file or with -m.

Why it matters

This is the first command a person runs. Getting a 400 on the first session after a successful setup reads as "the tool is broken", and nothing in the setup output points at the line that caused it.

The model also carries a second cost: codex warns Model metadata for 'gpt-5' not found. Defaulting to fallback metadata, so even where the name is accepted the session runs on guessed metadata.

Done when

• A project set up for Codex runs a session without being edited by hand.
• The written model is one the account can actually use, or no model is written at all and Codex's own default applies.
• A test fails when setup writes a model the tool would reject.

Open question, for whoever picks it up

Whether AIDD should pin a model here at all. Pinning means owning a name that changes outside this repository and differs by account type; not pinning means the tool decides. The measurement above only shows the current value is wrong, not which of the two is right.

Where it was found

While verifying #698 end to end on a real Codex session.