fix: exclude _platform directory from secret scanning in validator

The reusable validate workflow checks out the platform repo into
_platform/, causing false positives on rotate-credentials.sh and
test fixtures.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: baijum

validator/validate.py

@@ -254,7 +254,7 @@ def _check_secrets(self):
             rel_root = os.path.relpath(root, self.app_dir)
             skip = False
             for part in rel_root.split(os.sep):
-                if part.startswith(".") or part in ("node_modules", "__pycache__", "venv", ".venv"):
+                if part.startswith(".") or part in ("node_modules", "__pycache__", "venv", ".venv", "_platform"):
                     skip = True
                     break
             if skip: