docs: add agent context for oagen-generated code

Adds .claude/CLAUDE.md (with an AGENTS.md symlink) explaining that most
of this SDK is generated by the oagen pipeline from workos/openapi-spec,
how to tell generated files from hand-maintained ones, and that fixes to
generated code belong upstream in oagen/oagen-emitters/openapi-spec.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>