- Add model override fields to Task type and TaskStore (modelProvider, modelId, validatorModelProvider, validatorModelId)
- Extend PATCH /api/tasks/:id endpoint with validation for model fields
- Create ModelSelectorTab component with provider/model dropdowns and tests
- Integrate Model tab into TaskDetailModal for per-task model configuration
- Update executor to use per-task model overrides when both provider and modelId are set
- Update reviewer to use per-task validator model overrides in reviewStep
- Document per-task model selection feature in AGENTS.md settings section
