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>

schema/dialect/sqlserver/sqlserver_test.go

@@ -4,8 +4,8 @@ import (
 	"os"
 	"testing"
 
-	"git.fsdpf.net/go/db/v2/engine"
-	"git.fsdpf.net/go/db/v2/schema"
+	"git.fsdpf.net/go/db/engine"
+	"git.fsdpf.net/go/db/schema"
 	"github.com/stretchr/testify/suite"
 
 	_ "github.com/denisenkom/go-mssqldb"