Implement BoardScope fluent builder

- BoardScope struct for project-scoped operations
- Client.Board(projectID) returns a BoardScope
- Methods: GetColumns, GetCategories, GetTasks, SearchTasks, CreateTask
- All methods delegate to direct Client methods
- CreateTask automatically sets project ID from scope
- Comprehensive test coverage

board_scope.go

@@ -0,0 +1,44 @@
+package kanboard
+
+import "context"
+
+// BoardScope provides fluent project-scoped operations.
+type BoardScope struct {
+	client    *Client
+	projectID int
+}
+
+// Board returns a BoardScope for fluent project-scoped operations.
+func (c *Client) Board(projectID int) *BoardScope {
+	return &BoardScope{
+		client:    c,
+		projectID: projectID,
+	}
+}
+
+// GetColumns returns all columns for the project, sorted by position.
+func (b *BoardScope) GetColumns(ctx context.Context) ([]Column, error) {
+	return b.client.GetColumns(ctx, b.projectID)
+}
+
+// GetCategories returns all categories for the project.
+func (b *BoardScope) GetCategories(ctx context.Context) ([]Category, error) {
+	return b.client.GetAllCategories(ctx, b.projectID)
+}
+
+// GetTasks returns all tasks for the project with the specified status.
+func (b *BoardScope) GetTasks(ctx context.Context, status TaskStatus) ([]Task, error) {
+	return b.client.GetAllTasks(ctx, b.projectID, status)
+}
+
+// SearchTasks searches for tasks in the project using Kanboard query syntax.
+func (b *BoardScope) SearchTasks(ctx context.Context, query string) ([]Task, error) {
+	return b.client.SearchTasks(ctx, b.projectID, query)
+}
+
+// CreateTask creates a new task in the project.
+// The ProjectID field in the request is overwritten with the board's project ID.
+func (b *BoardScope) CreateTask(ctx context.Context, req CreateTaskRequest) (*Task, error) {
+	req.ProjectID = b.projectID
+	return b.client.CreateTask(ctx, req)
+}