fix: use defaultThinkingLevel from settings when enabledModels lacks explicit suffix (#540)

When enabledModels is configured without thinking level suffixes (e.g.,
'claude-opus-4-5' instead of 'claude-opus-4-5:high'), the scoped model's
default 'off' thinking level was overriding defaultThinkingLevel from
settings.

Now thinkingLevel in ScopedModel is optional (undefined means 'not
explicitly specified'). When passing to SDK, undefined values are filled
with defaultThinkingLevel from settings.

packages/coding-agent/CHANGELOG.md

@@ -11,6 +11,10 @@
 - Extension UI dialogs (`ctx.ui.select()`, `ctx.ui.confirm()`, `ctx.ui.input()`) now support a `timeout` option that auto-dismisses the dialog with a live countdown display. Simpler alternative to `AbortSignal` for timed dialogs.
 - Extensions can now provide custom editor components via `ctx.ui.setEditorComponent((tui, theme, keybindings) => ...)`. Extend `CustomEditor` for full app keybinding support (escape, ctrl+d, model switching, etc.). See `examples/extensions/modal-editor.ts`, `examples/extensions/rainbow-editor.ts`, and `docs/tui.md` Pattern 7.
 
+### Fixed
+
+- Default thinking level from settings now applies correctly when `enabledModels` is configured. Previously, models without explicit thinking level suffixes (e.g., `claude-opus-4-5` instead of `claude-opus-4-5:high`) would override `defaultThinkingLevel` with "off"
+
 ## [0.37.8] - 2026-01-07
 
 ## [0.37.7] - 2026-01-07