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>

sql_dialect_internal_test.go

@@ -3,9 +3,9 @@ package db
 import (
 	"testing"
 
-	"git.fsdpf.net/go/db/v2/exp"
-	"git.fsdpf.net/go/db/v2/internal/sb"
-	"git.fsdpf.net/go/db/v2/sqlgen/mocks"
+	"git.fsdpf.net/go/db/exp"
+	"git.fsdpf.net/go/db/internal/sb"
+	"git.fsdpf.net/go/db/sqlgen/mocks"
 	"github.com/stretchr/testify/suite"
 )