Implement TaskScope fluent builder (core methods)

- TaskScope struct for task-scoped operations
- Client.Task(taskID) returns a TaskScope
- Methods: Get, Close, Open, MoveToColumn, MoveToProject, Update
- MoveToColumn fetches task to get project_id and swimlane_id
- All methods delegate to direct Client methods
- Comprehensive test coverage

task_scope.go

@@ -0,0 +1,53 @@
+package kanboard
+
+import "context"
+
+// TaskScope provides fluent task-scoped operations.
+type TaskScope struct {
+	client *Client
+	taskID int
+}
+
+// Task returns a TaskScope for fluent task-scoped operations.
+func (c *Client) Task(taskID int) *TaskScope {
+	return &TaskScope{
+		client: c,
+		taskID: taskID,
+	}
+}
+
+// Get returns the task.
+func (t *TaskScope) Get(ctx context.Context) (*Task, error) {
+	return t.client.GetTask(ctx, t.taskID)
+}
+
+// Close closes the task (sets it to inactive).
+func (t *TaskScope) Close(ctx context.Context) error {
+	return t.client.CloseTask(ctx, t.taskID)
+}
+
+// Open opens the task (sets it to active).
+func (t *TaskScope) Open(ctx context.Context) error {
+	return t.client.OpenTask(ctx, t.taskID)
+}
+
+// MoveToColumn moves the task to a different column.
+// The task is placed at the end of the column (position=0).
+// Requires the project ID to be fetched from the task.
+func (t *TaskScope) MoveToColumn(ctx context.Context, columnID int) error {
+	task, err := t.Get(ctx)
+	if err != nil {
+		return err
+	}
+	return t.client.MoveTaskPosition(ctx, int(task.ProjectID), t.taskID, columnID, 0, int(task.SwimlaneID))
+}
+
+// MoveToProject moves the task to a different project.
+func (t *TaskScope) MoveToProject(ctx context.Context, projectID int) error {
+	return t.client.MoveTaskToProject(ctx, t.taskID, projectID)
+}
+
+// Update updates the task using TaskUpdateParams.
+func (t *TaskScope) Update(ctx context.Context, params *TaskUpdateParams) error {
+	return t.client.UpdateTaskFromParams(ctx, t.taskID, params)
+}