docs(oidc): add PKCE implementation and validation documentation
- Add plans/oidc_pkcd.md with phased implementation plan
- Add plans/oidc_pkce_validation.md with QA step-by-step local validation (Keycloak + Quay)
Documentation includes:
- Implementation phases and considerations
- Local development setup with Keycloak
- Step-by-step validation procedures
- Configuration examples and troubleshooting
Topic: pkce-docs
