SILENCE_SKIP_AGENTS hardcoded allowlist has rotted (missing scrp-*/claude-global)

hub.mjs:6438 SILENCE_SKIP_AGENTS is a hardcoded non-shim allowlist for the silence reaper (elazar-..., llmmsg-srv-hub, wi-cli, draandreaesparza-com). Missing all 8 scrp-* emitters and claude-global enumerated during tonight's stale-client work (coder-chatduo-cc, pmmaster-evolutiva-cc). Same name-list rot problem the caller_kind fix (v2.44.54) was built to avoid, and it has already rotted: agents that should be silence-exempt are not, or vice versa. Fix: derive from roster.last_poll_at (added in v2.44.54) instead of a name list, same pattern as /stale_clients caller_kind. Own diff + own gate — this changes alerting/paging behavior, not just a query endpoint.