F031's role_for_slug_or_none fix made the unknown-assignee release branch
in _dispatch_claimed_without_agent reachable (the human-only guard no
longer raises/short-circuits on a stale slug). Lock that reachability in:
a claimed task with an unknown-assignee UUID past grace returns the slug
(not None) so get_agent_role -> 'unknown' releases the claim to pending
for a role-matched reclaim.