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>

go.mod

@@ -1,4 +1,4 @@
-module git.fsdpf.net/go/db/v2
+module git.fsdpf.net/go/db
 
 go 1.19
 
@@ -18,6 +18,7 @@ require (
 	github.com/golang-sql/sqlexp v0.1.0 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/stretchr/objx v0.5.2 // indirect
+	github.com/ugorji/go/codec v1.2.12 // indirect
 	golang.org/x/crypto v0.11.0 // indirect
 	golang.org/x/text v0.21.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect