[SizzleUnrlsd]

Summary

Third item of #71: credentials in test code and template files are reported at low confidence.

• Acceptance tests first (test: define the confidence of credentials in tests and examples), implementation follows.
• plugins/secrets.py: credential_context classifies a path as test (a tests, test, testing, fixtures or __tests__ directory, a test_*.py, *_test.py or conftest.py file), example (.example, .sample, .dist, .tpl suffixes) or neither. A hardcoded-credential there is reported at low confidence with the context in its metadata; elsewhere it stays medium. Provider-format hardcoded-secret findings keep their confidence everywhere, since a real key in a test is still a key.
• On healthchecks, 37 of the 40 credential findings sit in tests and one in .env.example; a consumer sorting or filtering on confidence now sees the three that matter first. Snapshots record rule and location only, so they are unchanged.
• The usage guide documents the rule.

Part of #71.