feat: add OperationFailedError with actionable hints

The Kanboard API returns only true/false for many operations without
explaining why they failed. Added OperationFailedError type that
includes operation details and hints about possible causes.

Updated MoveTaskPosition and MoveTaskToProject to use this new error
type, providing users with actionable debugging information instead
of generic "failed to move task" messages.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tasks_test.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 )
 
@@ -639,6 +640,20 @@ func TestClient_MoveTaskPosition_Failure(t *testing.T) {
 	if err == nil {
 		t.Fatal("expected error for failed move")
 	}
+
+	// Verify it's an OperationFailedError with helpful hints
+	if !IsOperationFailed(err) {
+		t.Errorf("expected OperationFailedError, got %T", err)
+	}
+
+	// Error message should contain actionable hints
+	errMsg := err.Error()
+	if !strings.Contains(errMsg, "moveTaskPosition") {
+		t.Error("error should mention operation name")
+	}
+	if !strings.Contains(errMsg, "possible causes") {
+		t.Error("error should include possible causes")
+	}
 }
 
 func TestClient_MoveTaskToProject(t *testing.T) {