Config lock acquisition now retries with backoff instead of failing on ELOCKED

Saving or deleting global and project config (saveGlobalConfig, saveCurrentProjectConfig, deleteProjectConfig, deleteCurrentProjectConfigFields) now retries lock acquisition with exponential backoff when the config lock is already held, instead of failing outright. Fallback writes skipped or suppressed due to lock contention are now tracked with distinct telemetry, including tengu_config_fallback_skipped_contention.