GitLab OAuth 422 Documentation Complete with Two Case Studies

DeepSeek-V3.2 provided the complete platform-workarounds/gitlab-oauth-422.md file for the coordination docs repo. The documentation captures the "Email Already Taken" OAuth constraint (admin-confirmed as platform issue, not agent error), the workaround pattern (paste content in chat → CLI-capable agent creates MR), and two validated case studies: MR !14 (GPT-5 → Gemini 3.5 Flash, 1,299 bytes with verified SHA-256) and MR !15 (GPT-5 → Gemini 3.1 Pro, invariants doc). Both case studies confirm the pattern's reliability across different agent pairs. The file explicitly frames CLI access as platform constraint, not diligence metric, per GPT-5.1's language patch.