Add Exercise To Workout (from Workout Detail)

@ivanscorral is already working on this.

Since Oct 12, 2025.

### Summary
Wire the **plus button** in `KoWorkout/Views/WorkoutDetailView.swift:18-23` to add an exercise to the current workout by selecting from the exercise library (and optionally creating a new exercise).

---

### Data Layer
Add `WorkoutRepository` and `InMemoryWorkoutRepository` to manage `Workout` mutations (align with `ExerciseRepository`).

**Protocol (minimal):**
```swift
@MainActor
protocol WorkoutRepository: AnyObject {
func get(_ id: UUID) -> Workout?
func update(_ id: UUID, _ mutate: (inout Workout) -> Void)
}
```

### Convenience:
```swift
func addExercise(to id: UUID, exercise: Exercise, notes: String? = nil, order: Int? = nil)
```

> **Note:** A repository is required because `WorkoutDetailViewModel` currently owns a value copy of `Workout` and has no mutation pathways.

---

### ViewModel
Update `WorkoutDetailViewModel` to:
- Accept `workoutId: UUID`, `workoutRepository: WorkoutRepository`, and `exerciseRepository: ExerciseRepository`.
- Add state `isPresentingExercisePicker: Bool` and action `presentExercisePicker()`.
- Add action `addExercise(_ exercise: Exercise)` that:
- Calls `workoutRepository.addExercise(to: workoutId, exercise: exercise)`
- Refreshes display state from repository.
- Update initializer and `updateWorkout(_:)` to hydrate from repository.

---

### UI/UX
From `WorkoutDetailView.swift:18-23`:
- Present a **sheet** when tapping the plus button — an `ExercisePickerView` (new) that lists library exercises using `ExerciseListViewModel` / `ExerciseRepository`.

**In the picker:**
- Selecting an existing exercise → calls `viewModel.addExercise(selectedExercise)`
- Creating a new exercise via `AddExerciseView` → select it manually or auto-select on save.

---

### Acceptance Criteria
- ✅ Tapping **plus** opens a picker
- ✅ Choosing an exercise inserts it into the workout and updates the UI immediately (in correct order)
- ✅ When the workout has no exercises, adding one hides the empty state (`KoWorkout/Views/WorkoutDetailView.swift:233-241`)
- ✅ Previews continue to compile with an in-memory repository (provide preview fixtures)

---

### Files To Touch
- `KoWorkout/Views/WorkoutDetailView.swift:18-23`
- `KoWorkout/ViewModels/WorkoutDetailViewModel.swift:12` (constructor), `:69` (state), and add new actions
- **New:** `KoWorkout/Repositories/WorkoutRepository.swift`
- **New:** `KoWorkout/Views/ExercisePickerView.swift` *(or adapt `ExerciseListView` to support selection mode)*