KI-007 (High): Add isinstance(value, str) guard to _is_hex() so
non-string arguments return False cleanly instead of raising TypeError.
Updated test_is_hex_non_string to assert False instead of expecting
pytest.raises(TypeError).
KI-005 (High): Add _scan_for_secrets() to manifest.py that walks all
string values in plugin.yaml and reports common credential patterns
(sk-, ghp_, AKIA, bearer tokens, long hex strings, password/api_key
assignments). Call it from validate_manifest(). Skips the sha256
field since it's a content-addressed hash, not a secret.
Run: pytest → 210 passed, 1 skipped.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>