The poller read `<state>/github-token` once at start, so after a PAT
rotation it kept polling with the revoked token until the unit
restarted. It now re-reads the file on every tick, as
`hive-forge-notify` does: a different non-empty value rebuilds the
GitHub source, clears the own-login cache, and logs one info line. A
missing or empty file keeps the token in use.
A 401 from the notifications endpoint is now logged at warn, once per
token value; other non-2xx statuses stay at debug.
Part of #5017 (slice G).
cf763da178