Add Codex Ultrafast public beta support

[apollostreetcompany]

What this adds

RepoPrompt now shows an Ultrafast — Public Beta option for Codex models that say they support it.

When someone chooses Ultrafast, RepoPrompt keeps that choice when:

• starting a new session
• reopening an existing session
• sending later messages in the same session

The exact ultrafast setting is sent to Codex instead of being dropped or replaced.

What stays the same

• Ultrafast only appears when the Codex model list says it is supported.
• Models such as codex-auto-review do not get an Ultrafast option.
• The existing Fast option still works as before.
• Choosing no speed option still uses the normal Codex default.
• If Codex rejects Ultrafast, RepoPrompt shows that error instead of quietly switching models.
• Ultrafast keeps the model's recommended reasoning level unless the user chooses another one.

This PR does not add a setting for forcing Ultrafast on unsupported models. That is separate follow-up work.

Testing

The focused model-picker and Codex session tests pass. They cover:

• which models show Ultrafast
• matching choices across RepoPrompt's model pickers
• new, reopened, and continuing sessions
• unchanged Fast and default behavior
• exact error handling without fallback
• keeping the model's recommended reasoning level

Formatting, linting, the Codex schema check, the RepoPrompt build, test-ledger verification, repository guardrails, secret scanning, and contribution checks also pass.

The required GPT-5.6 Sol High review found one issue: the first version could forget the model's recommended reasoning level for Ultrafast. Follow-up commit 2dae4eaf72e083d3825f88909b79f658830611b9 fixes it, the focused tests pass, and the follow-up review is clean.

Known validation notes

• A full local test run was stopped after more than 23 minutes because unrelated worktree and Context Builder tests were failing or hanging. No Ultrafast-focused test failed.
• The non-disruptive live smoke check could not run because the local debug command-line tool is not installed. The app was not launched or stopped.
• Hosted checks are running on the latest commit. This PR should remain a draft until those checks finish.