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_example_test.go

@@ -5,9 +5,9 @@ import (
 	dbsql "database/sql"
 	"fmt"
 
-	dbv2 "git.fsdpf.net/go/db/v2"
-	_ "git.fsdpf.net/go/db/v2/dialect/mysql"
-	"git.fsdpf.net/go/db/v2/exp"
+	dbv2 "git.fsdpf.net/go/db"
+	_ "git.fsdpf.net/go/db/dialect/mysql"
+	"git.fsdpf.net/go/db/exp"
 )
 
 func ExampleUpdate_withStruct() {