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/mysql/mysql_builder_test.go

@@ -3,9 +3,9 @@ package mysql_test
 import (
 	"os"
 
-	"git.fsdpf.net/go/db/v2"
-	"git.fsdpf.net/go/db/v2/engine"
-	"git.fsdpf.net/go/db/v2/schema"
+	"git.fsdpf.net/go/db"
+	"git.fsdpf.net/go/db/engine"
+	"git.fsdpf.net/go/db/schema"
 	"github.com/stretchr/testify/suite"
 )
 
@@ -40,7 +40,7 @@ func (t *mysqlBuilderTest) TestGetColumnListing() {
 // 创建表
 func (t *mysqlBuilderTest) TestCreateTable() {
 	if err := t.builder.Create("users", func(table *schema.Blueprint) {
-		table.Charset("utf8mb4")
+		table.Charset = "utf8mb4"
 
 		table.BigIncrements("id").Comment("ID")
 		table.Boolean("enabled").Default("1").Comment("是否有效")