feat(kanban): gate notifier watcher on dispatch_in_gateway

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.

gateway/run.py

@@ -5121,6 +5121,30 @@ class GatewayRunner:
         cross boards, so delivery semantics are unchanged — this is
         purely a fan-out of the single-DB poll.
         """
+        # Gate: only the dispatch-owning gateway opens kanban DBs for notifier polling.
+        # Non-dispatch gateways have no subscriptions to deliver — all kanban state lives
+        # in the dispatch owner's per-board DBs. This prevents N-gateway -shm contention.
+        # TODO: gate per-board when per-board dispatcher_owner tracking lands.
+        try:
+            from hermes_cli.config import load_config as _load_config
+        except Exception:
+            logger.warning("kanban notifier: config loader unavailable; disabled")
+            return
+        env_override = os.environ.get("HERMES_KANBAN_DISPATCH_IN_GATEWAY", "").strip().lower()
+        if env_override in {"0", "false", "no", "off"}:
+            logger.info("kanban notifier: disabled via HERMES_KANBAN_DISPATCH_IN_GATEWAY env")
+            return
+        try:
+            cfg = _load_config()
+        except Exception as exc:
+            logger.warning("kanban notifier: cannot load config (%s); disabled", exc)
+            return
+        kanban_cfg = cfg.get("kanban", {}) if isinstance(cfg, dict) else {}
+        if not kanban_cfg.get("dispatch_in_gateway", True):
+            logger.info(
+                "kanban notifier: disabled via config kanban.dispatch_in_gateway=false"
+            )
+            return
         from gateway.config import Platform as _Platform
         try:
             from hermes_cli import kanban_db as _kb