Non-dispatch gateways no longer open per-board kanban DBs for notifier
polling. Mirrors the existing dispatcher gate (config
kanban.dispatch_in_gateway, default True; env override
HERMES_KANBAN_DISPATCH_IN_GATEWAY) so multi-gateway setups collapse to a
single process holding kanban.db file descriptors.
Salvaged from PR #31964 by @steveonjava; tests and docs trimmed during
salvage.
