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>

exp/update.go

@@ -4,8 +4,8 @@ import (
 	"reflect"
 	"sort"
 
-	"git.fsdpf.net/go/db/v2/internal/errors"
-	"git.fsdpf.net/go/db/v2/internal/util"
+	"git.fsdpf.net/go/db/internal/errors"
+	"git.fsdpf.net/go/db/internal/util"
 )
 
 type (