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>

errors_test.go

@@ -3,6 +3,7 @@ package kanboard
 import (
 	"errors"
 	"fmt"
+	"strings"
 	"testing"
 )
 
@@ -217,3 +218,71 @@ func TestErrorWrapping(t *testing.T) {
 		t.Errorf("unexpected error message: %s", wrappedTwice.Error())
 	}
 }
+
+func TestOperationFailedError(t *testing.T) {
+	tests := []struct {
+		name           string
+		err            *OperationFailedError
+		expectedSubstr []string
+	}{
+		{
+			name: "with hints",
+			err: &OperationFailedError{
+				Operation: "moveTaskPosition(task=42, column=5, project=1)",
+				Hints:     []string{"task may not exist", "column may not belong to project"},
+			},
+			expectedSubstr: []string{
+				"moveTaskPosition",
+				"operation failed",
+				"possible causes",
+				"task may not exist",
+				"column may not belong to project",
+			},
+		},
+		{
+			name: "without hints",
+			err: &OperationFailedError{
+				Operation: "someOperation",
+				Hints:     nil,
+			},
+			expectedSubstr: []string{"someOperation", "operation failed"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			errMsg := tt.err.Error()
+			for _, substr := range tt.expectedSubstr {
+				if !containsSubstr(errMsg, substr) {
+					t.Errorf("error message %q should contain %q", errMsg, substr)
+				}
+			}
+		})
+	}
+}
+
+func TestIsOperationFailed(t *testing.T) {
+	tests := []struct {
+		name     string
+		err      error
+		expected bool
+	}{
+		{"OperationFailedError", &OperationFailedError{Operation: "test"}, true},
+		{"wrapped OperationFailedError", fmt.Errorf("call failed: %w", &OperationFailedError{Operation: "test"}), true},
+		{"ErrUnauthorized", ErrUnauthorized, false},
+		{"generic error", errors.New("some error"), false},
+		{"nil", nil, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := IsOperationFailed(tt.err); got != tt.expected {
+				t.Errorf("IsOperationFailed(%v) = %v, want %v", tt.err, got, tt.expected)
+			}
+		})
+	}
+}
+
+func containsSubstr(s, substr string) bool {
+	return strings.Contains(s, substr)
+}