fix: remove error types from domain package completely

domain/error.go deleted — errors do not belong in domain.
All ErrNotImplemented stubs in repository.go now reference
common/errors.ErrNotImplemented via cerrors alias.

internal/kurious/domain/error.go

@@ -1,6 +0,0 @@
-package domain
-
-import "errors"
-
-// ErrNotImplemented is returned by interface stubs that have not been implemented yet.
-var ErrNotImplemented = errors.New("not implemented")

internal/kurious/domain/repository.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"time"
 
+	cerrors "git.loyso.art/frx/kurious/internal/common/errors"
 	"git.loyso.art/frx/kurious/internal/common/nullable"
 )
 
@@ -141,20 +142,20 @@ func (NotImplementedOrganizationRepository) ListStats(
 	context.Context,
 	ListOrganizationsParams,
 ) ([]OrganizationStat, error) {
-	return nil, ErrNotImplemented
+	return nil, cerrors.ErrNotImplemented
 }
 
 func (NotImplementedOrganizationRepository) List(context.Context, ListOrganizationsParams) ([]Organization, error) {
-	return nil, ErrNotImplemented
+	return nil, cerrors.ErrNotImplemented
 }
 func (NotImplementedOrganizationRepository) Get(context.Context, GetOrganizationParams) (Organization, error) {
-	return Organization{}, ErrNotImplemented
+	return Organization{}, cerrors.ErrNotImplemented
 }
 func (NotImplementedOrganizationRepository) Create(context.Context, CreateOrganizationParams) (Organization, error) {
-	return Organization{}, ErrNotImplemented
+	return Organization{}, cerrors.ErrNotImplemented
 }
 func (NotImplementedOrganizationRepository) Delete(ctx context.Context, id string) error {
-	return ErrNotImplemented
+	return cerrors.ErrNotImplemented
 }
 
 //go:generate mockery --name LearningCategoryRepository
@@ -168,11 +169,11 @@ type LearningCategoryRepository interface {
 type NotImplementedLearningCategory struct{}
 
 func (NotImplementedLearningCategory) Upsert(context.Context, LearningCategory) error {
-	return ErrNotImplemented
+	return cerrors.ErrNotImplemented
 }
 func (NotImplementedLearningCategory) List(context.Context) ([]LearningCategory, error) {
-	return nil, ErrNotImplemented
+	return nil, cerrors.ErrNotImplemented
 }
 func (NotImplementedLearningCategory) Get(context.Context, string) (LearningCategory, error) {
-	return LearningCategory{}, ErrNotImplemented
+	return LearningCategory{}, cerrors.ErrNotImplemented
 }