MSG-332d: 4min timeout, single straggler DM, then liveness check

Entry not complete after N minutes (default 4, Elazar-set this session) triggers ONE straggler DM asking status - not a broadcast. If no reply, fall through to the fleet agent-liveness diagnostic to distinguish idle/compacting/wedged/dead before treating it as a real block. Do NOT hardcode the diagnostic script name: ask bin-venus-cc for the current canonical path, it has moved before. Open design question to settle in this WI: does the straggler DM come from the hub or from the opener - hub keeps it off the opener's token budget, opener keeps the hub out of the conversation.