docs: Add CLAUDE.md with codebase guidance

Create comprehensive documentation for future Claude Code instances working in this repository, including:
- Development commands for testing, building, and code quality
- Core architecture overview of the SQL query builder system
- Directory structure and component explanations
- Testing patterns and conventions
- Key dependencies and their purposes

🤖 Generated with [Claude Code](https://claude.ai/code)

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

update_dataset_test.go

@@ -3,11 +3,11 @@ package db_test
 import (
 	"testing"
 
-	dbv2 "git.fsdpf.net/go/db/v2"
-	"git.fsdpf.net/go/db/v2/exp"
-	"git.fsdpf.net/go/db/v2/internal/errors"
-	"git.fsdpf.net/go/db/v2/internal/sb"
-	"git.fsdpf.net/go/db/v2/mocks"
+	dbv2 "git.fsdpf.net/go/db"
+	"git.fsdpf.net/go/db/exp"
+	"git.fsdpf.net/go/db/internal/errors"
+	"git.fsdpf.net/go/db/internal/sb"
+	"git.fsdpf.net/go/db/mocks"
 	"github.com/DATA-DOG/go-sqlmock"
 	"github.com/stretchr/testify/mock"
 	"github.com/stretchr/testify/suite"