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>

sqlgen/mocks/TruncateSQLGenerator.go

@@ -2,9 +2,9 @@
 
 package mocks
 
-import exp "git.fsdpf.net/go/db/v2/exp"
+import exp "git.fsdpf.net/go/db/exp"
 import mock "github.com/stretchr/testify/mock"
-import sb "git.fsdpf.net/go/db/v2/internal/sb"
+import sb "git.fsdpf.net/go/db/internal/sb"
 
 // TruncateSQLGenerator is an autogenerated mock type for the TruncateSQLGenerator type
 type TruncateSQLGenerator struct {