ci: add nightly external contract tests

README.md

@@ -102,6 +102,14 @@ cargo test --locked
 
 The crate denies unsafe code and enables strict Clippy groups: `all`, `pedantic`, `nursery` and `cargo`, with only dependency-version noise allowed.
 
+## External contract checks
+
+Normal tests mock third-party APIs. A scheduled GitHub Actions workflow runs ignored contract tests nightly against the real PokéAPI and FunTranslations APIs to catch provider contract drift early:
+
+```bash
+cargo test --locked --test external_contract -- --ignored --test-threads=1
+```
+
 ## Docker multi-arch build
 
 ```bash