StartupValidator warned "Plugin 'auto_update' is enabled but not found" on
every display start with auto-update or a dim schedule on; the reserved
plugin-id check missed auto_update, sync, location and the rest; and
ConfigManager's (uncalled) orphan cleanup would have deleted display,
schedule and auto_update. All three now read src/core_config_keys.py, which
also gains CORE_SECRETS_KEYS for the github/youtube secrets sections.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>