Prevent arbitrary file write and repo traversal by strictly validating
`github_repo` and `file_path` inputs in `update_github_cdn_urls`.
Added `validate_github_repo` and `validate_file_path` functions to
enforce strict whitelisting of characters and reject path traversal sequences.
Added comprehensive unit tests in `tests/test_github_validation.py`.
- Sanitize `DiscordWebhookClient` exception messages to redact webhook tokens.
- Sanitize GitHub API response text in error messages to prevent token leakage.
- Update `tests/test_utils.py` with mocks and new security test cases.
- Record security learning in `.jules/sentinel.md`.
This addresses potential credential exposure in logs and error messages.