OWNER: add PostgreSQL-only governance

Author: ToolboxAid

docs_build/dev/ProjectInstructions/addendums/postgres_only.md

@@ -0,0 +1,11 @@
+SQLite is deprecated.
+
+Game Foundry Studio standard database:
+PostgreSQL
+
+Rules:
+- No new SQLite implementations
+- No new SQLite dependencies
+- Existing SQLite references are technical debt
+- New persistence work targets PostgreSQL
+- PRs introducing SQLite should be rejected