Implement Column API methods (GetColumns, GetColumn)

- GetColumns returns all columns for a project, sorted by position
- GetColumn returns a single column by ID
- Returns ErrColumnNotFound for non-existent columns
- Comprehensive test coverage including edge cases

columns.go

@@ -0,0 +1,41 @@
+package kanboard
+
+import (
+	"context"
+	"fmt"
+	"sort"
+)
+
+// GetColumns returns all columns for a project, sorted by position.
+func (c *Client) GetColumns(ctx context.Context, projectID int) ([]Column, error) {
+	params := map[string]int{"project_id": projectID}
+
+	var result []Column
+	if err := c.call(ctx, "getColumns", params, &result); err != nil {
+		return nil, fmt.Errorf("getColumns: %w", err)
+	}
+
+	// Sort by position
+	sort.Slice(result, func(i, j int) bool {
+		return int(result[i].Position) < int(result[j].Position)
+	})
+
+	return result, nil
+}
+
+// GetColumn returns a column by its ID.
+// Returns ErrColumnNotFound if the column does not exist.
+func (c *Client) GetColumn(ctx context.Context, columnID int) (*Column, error) {
+	params := map[string]int{"column_id": columnID}
+
+	var result *Column
+	if err := c.call(ctx, "getColumn", params, &result); err != nil {
+		return nil, fmt.Errorf("getColumn: %w", err)
+	}
+
+	if result == nil {
+		return nil, fmt.Errorf("%w: column %d", ErrColumnNotFound, columnID)
+	}
+
+	return result, nil
+}