fix(codex): drop the settings panel's Reasoning Effort control

The Codex panel offered a Reasoning Effort dropdown that wrote the root-level
`model_reasoning_effort` into config.toml. It was never the lever it looked
like: the effort a turn actually runs at comes from the composer's own
per-session picker, which codex exposes over ACP as a session config option, so
the panel's value was a launch-time default that the visible picker routinely
disagreed with.

The key was not opt-in either. `patchCodexConfigTomlText` rewrote it at the end
of every patch, with "high" standing in whenever the file declared nothing, so
flipping the WebSocket switch or editing a model name was enough to plant
`model_reasoning_effort = "high"` in a config that had never mentioned it,
overriding whatever per-model default codex would otherwise have applied.

The control goes and the whole read/write chain behind it goes with it. Hiding
the dropdown alone would have left that unconditional write in place, editing
config.toml with nothing on screen to account for it. A `model_reasoning_effort`
line already in a user's file is now left exactly as written, since nothing
targets that key any more; the raw config.toml editor below still carries it in
its placeholder and is where it gets set by hand. The per-model default in the
model catalog editor is a separate field and is untouched.

The panel's description drops its "reasoning effort" claim across all ten
locales, and the dropdown's placeholder string goes with the dropdown.