fix: doc-link warning in model_picker module header

Removed the `[`Settings`]` intra-doc link that referenced a type not
in scope at the module-doc level — RUSTDOCFLAGS=-Dwarnings rejects it.
Replaced with backtick code formatting; the rest of the doc is
unchanged.

Caught by the Documentation CI job on v0.5.2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/tui/src/tui/model_picker.rs

@@ -14,7 +14,7 @@
 //!
 //! On apply we emit a [`ViewEvent::ModelPickerApplied`] with the resolved
 //! model id and effort tier; the UI handler updates `App` state, persists
-//! the choice via [`Settings`], and forwards `Op::SetModel` so the running
+//! the choice via `Settings`, and forwards `Op::SetModel` so the running
 //! engine picks up the change without a restart.
 
 use crossterm::event::{KeyCode, KeyEvent};