## 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.
hefuc-oai
cf36c688b3