[codex] clarify frontend PKCE with server-side token exchange (#92)

* docs: clarify oauth client auth modes

* docs: align oauth guides with current request access flow

* docs: keep frontend oauth examples with backend exchange

* docs: clarify frontend and backend oauth wording

* docs: align oauth wording with qf provisioning flow

* docs: standardize oauth provisioning terminology

* docs: address oauth review feedback

Author: basit3407