feat: add CreateSubHabit AI action + refactor SystemPromptBuilder

src/Orbit.Application/Chat/Commands/ProcessUserChatCommand.cs

@@ -191,6 +191,7 @@ public async Task<Result<ChatResponse>> Handle(
                     AiActionType.AssignTags => await ExecuteAssignTagsAsync(action, request.UserId, cancellationToken),
                     AiActionType.UpdateHabit => await ExecuteUpdateHabitAsync(action, request.UserId, cancellationToken),
                     AiActionType.DeleteHabit => await ExecuteDeleteHabitAsync(action, request.UserId, cancellationToken),
+                    AiActionType.CreateSubHabit => await ExecuteCreateSubHabitAsync(action, request.UserId, cancellationToken),
                     _ => Result.Failure<(Guid? Id, string? Name)>($"Unknown action type: {action.Type}")
                 };
 
@@ -513,6 +514,27 @@ public async Task<Result<ChatResponse>> Handle(
         return Result.Success<(Guid? Id, string? Name)>((habit.Id, title));
     }
 
+    private async Task<Result<(Guid? Id, string? Name)>> ExecuteCreateSubHabitAsync(
+        AiAction action, Guid userId, CancellationToken ct)
+    {
+        if (action.HabitId is null)
+            return Result.Failure<(Guid? Id, string? Name)>("Parent habit ID is required for creating a sub-habit.");
+
+        if (string.IsNullOrWhiteSpace(action.Title))
+            return Result.Failure<(Guid? Id, string? Name)>("Title is required to create a sub-habit.");
+
+        var result = await mediator.Send(new Orbit.Application.Habits.Commands.CreateSubHabitCommand(
+            userId,
+            action.HabitId.Value,
+            action.Title,
+            action.Description), ct);
+
+        if (result.IsFailure)
+            return Result.Failure<(Guid? Id, string? Name)>(result.Error);
+
+        return Result.Success<(Guid? Id, string? Name)>((result.Value, action.Title));
+    }
+
     private async Task AssignTagsToHabitAsync(Habit habit, List<string>? tagNames, Guid userId, CancellationToken ct)
     {
         if (tagNames is not { Count: > 0 }) return;

src/Orbit.Domain/Enums/AiActionType.cs

@@ -7,5 +7,6 @@ public enum AiActionType
     SuggestBreakdown,
     AssignTags,
     UpdateHabit,
-    DeleteHabit
+    DeleteHabit,
+    CreateSubHabit
 }