Add test credentials and guidelines for staging environment in CLAUDE.md

CLAUDE.md

@@ -214,6 +214,15 @@ pnpm typecheck      # TypeScript type checking
 
 ## Development Tools & Testing
 
+### Test Credentials (Cidaas Staging)
+
+**📖 Complete test credentials and testing guide:** [`docs/TESTING.md`](./docs/TESTING.md)
+
+**Important:**
+- Test credentials are documented in [`docs/TESTING.md`](./docs/TESTING.md)
+- ⚠️ **Only for staging environment** - NEVER use in production
+- Used by Playwright E2E tests and Vitest integration tests
+
 ### Playwright MCP Server
 
 Ein Playwright MCP Server ist installiert und verfügbar für: