Remote startup waits for its credentials file instead of failing fast

A remote session that has to read its access token from a file now retries on a fixed backoff (250, 500, 500, 750, 1000 ms) and then waits a grace period for OAuth or API credentials to appear, rather than giving up immediately. It logs "[spare-claim] session ingress token file not readable yet" while waiting and reports attempts, whether it recovered, and total wait time.