[codex] Use managed defaults for TUI threads

[hefuc-oai]

Why

#29683 exposes managed defaults for new-thread model settings through configRequirements/read without applying them server-wide. The TUI is an app-server client, so it should explicitly consume those defaults when it creates a fresh thread.

This lets plain codex start on the managed model while preserving the existing ability to change model settings within the thread.

What changed

• Read requirements.models.newThread during TUI app-server bootstrap.
• Apply the managed model, reasoning effort, and service tier to the initial fresh thread and subsequent /new or /clear threads.
• Keep explicit launch overrides above the managed defaults.
• Normalize the managed fast service tier to the priority request value.
• Leave resumed and forked threads unchanged.

The application logic lives in a small TUI-only module; app-server thread/start behavior remains unchanged for other clients.

User experience

• Plain codex starts with the managed new-thread settings.
• A user can still change settings with /model or the existing service-tier controls.
• Starting another fresh thread reapplies the managed defaults.
• Explicit launch choices such as codex -m <model> continue to win.

Validation

• just test -p codex-tui managed_new_thread_defaults
• just fix -p codex-tui

Depends on #29683.

[chatgpt-codex-connector]

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