feat: add per-project default model override for new features (#640)

* feat: add per-project default model override for new features

- Add defaultFeatureModel to ProjectSettings type for project-level override
- Add defaultFeatureModel to Project interface for UI state
- Display Default Feature Model in Model Defaults section alongside phase models
- Include Default Feature Model in global Bulk Replace dialog
- Add Default Feature Model override section to Project Settings
- Add setProjectDefaultFeatureModel store action for project-level overrides
- Update clearAllProjectPhaseModelOverrides to also clear defaultFeatureModel
- Update add-feature-dialog to use project override when available
- Include Default Feature Model in Project Bulk Replace dialog

This allows projects with different complexity levels to use different
default models (e.g., Haiku for simple tasks, Opus for complex projects).

* fix: add server-side __CLEAR__ handler for defaultFeatureModel

- Add handler in settings-service.ts to properly delete defaultFeatureModel
  when '__CLEAR__' marker is sent from the UI
- Fix bulk-replace-dialog.tsx to correctly return claude-opus when resetting
  default feature model to Anthropic Direct (was incorrectly using
  enhancementModel's settings which default to sonnet)

These fixes ensure:
1. Clearing project default model override properly removes the setting
   instead of storing literal '__CLEAR__' string
2. Global bulk replace correctly resets default feature model to opus

* fix: include defaultFeatureModel in Reset to Defaults action

- Updated resetPhaseModels to also reset defaultFeatureModel to claude-opus
- Fixed initial state to use canonical 'claude-opus' instead of 'opus'

* refactor: use DEFAULT_GLOBAL_SETTINGS constant for defaultFeatureModel

Address PR review feedback:
- Replace hardcoded { model: 'claude-opus' } with DEFAULT_GLOBAL_SETTINGS.defaultFeatureModel
- Fix Prettier formatting for long destructuring lines
- Import DEFAULT_GLOBAL_SETTINGS from @automaker/types where needed

This improves maintainability by centralizing the default value.

apps/ui/src/lib/electron.ts

@@ -3412,6 +3412,11 @@ export interface Project {
    * If a phase is not present, the global setting is used.
    */
   phaseModelOverrides?: Partial<import('@automaker/types').PhaseModelConfig>;
+  /**
+   * Override the default model for new feature cards in this project.
+   * If not specified, falls back to the global defaultFeatureModel setting.
+   */
+  defaultFeatureModel?: import('@automaker/types').PhaseModelEntry;
 }
 
 export interface TrashedProject extends Project {