agents get a todo when assigned to an issue or PR, rewritten when unassigned and cleared on close

A new swarm service, swarm-notify-router, in its own container on the host
that sets deploy.notify-router.enable (defaulted on by allSwarmServices where
the host has the router's store leaf, like the claude login minter).

- swarm-controller registers instance-wide hooks for `issues` +
  `issue_assign` and `pull_request` + `pull_request_assign`, updates the
  event list of a hook already registered with fewer (leaving its active flag as it is), and keeps each
  verified body unchanged in the SWARM_EVENTS stream on
  $SWARM.events.forge.<X-Forgejo-Event-Type>, deduped by X-Forgejo-Delivery.
  A delivery the stream did not keep answers 503.
- swarm-notify-router drains its durable consumer on that stream, runs its
  flows (source -> match -> todo sink, stored as nodes plus edges and checked
  to be a tree) and queues each op on $SWARM.router.todo.<agent> in the
  work-queue stream ROUTER_TODOS. Built-in flows: assigned, unassigned (same
  key, rewritten) and closed (prefix-clears every holder's router todos). A
  per-issue snapshot in its notify-router-state bucket holds the last
  assignees and the todo holders; only agents in the agent-status bucket get
  ops, and an assignee that was not one is left out of the snapshot so the
  next assign event retries it. After the first event that must be retried,
  the rest of the batch is nakked so redelivery keeps stream order.
- hive-agent creates its own durable consumer todo-<agent> and applies each
  op to its todo store as subsystem `router`, then acks; a refused op stops
  the rest of its batch the same way. The store's (subsystem, key) dedup
  makes a redelivery harmless. New Todos::clear_prefix.
- swarm-nats-auth: a --notify-router-client arm; the controller may publish
  $SWARM.events.forge.*; each agent gets three {agent} templates (extended
  consumer create whose filter the server checks, MSG.NEXT, ACK).
- Credentials: a notify-router store leaf minted by glue-bao-tls, a
  swarm-notify-router bao policy (read on its own queue client secret), an
  authelia machine client, the secret publisher entry, a journal forwarder
  route. No manual step.

hive-forge-notify is unchanged, so agents also still get forge's own
notifications.

Refs #5571

nix/host-modules/swarm-bao.nix

@@ -1148,6 +1148,16 @@ let
     }
   ];
 
+  # The notification router: its own queue client secret, read at start by the
+  # daemon inside its container, and nothing else.
+  notifyRouterReaders = [
+    {
+      name = "swarm-notify-router";
+      cn = baoDeploy.notifyRouterCommonName;
+      policyText = readStanza "${credentialMountPath}/data/swarm/services/${hyperhiveCfg.swarm.notify-router.clientId}/oidc/client";
+    }
+  ];
+
   # The forge host's one store identity. The forge's
   # `forgejo-swarm-controller-account`, in its container, writes the
   # controller's forge token under it, the one writer of that path and
@@ -2401,6 +2411,18 @@ in
       '';
     };
 
+    notifyRouterCommonName = lib.mkOption {
+      type = lib.types.str;
+      default = "swarm-notify-router";
+      description = ''
+        Subject the store's `swarm-notify-router` cert-auth role accepts: the
+        identity the notification router presents from inside its container.
+        Its grant is read on its own queue client secret.
+
+        ⚠️ Reserved as a hive name by ./swarm.nix, like its siblings.
+      '';
+    };
+
     claudeLoginClientCertFile = lib.mkOption {
       type = lib.types.nullOr lib.types.str;
       default = null;
@@ -2426,6 +2448,31 @@ in
       '';
     };
 
+    notifyRouterClientCertFile = lib.mkOption {
+      type = lib.types.nullOr lib.types.str;
+      default = null;
+      example = "/var/lib/swarm-bao-pki/notify-router.pem";
+      description = ''
+        Certificate the notification router presents to the store. Its subject
+        must be {option}`services.hyperhive.deploy.bao.notifyRouterCommonName`.
+
+        ./glue-bao-tls.nix points it at the leaf it mints on the store's host.
+        On a router host without the store, it is a file the operator copies
+        from the store's host. A path, never a value.
+      '';
+    };
+
+    notifyRouterClientKeyFile = lib.mkOption {
+      type = lib.types.nullOr lib.types.str;
+      default = null;
+      example = "/var/lib/swarm-bao-pki/notify-router-key.pem";
+      description = ''
+        Private key for
+        {option}`services.hyperhive.deploy.bao.notifyRouterClientCertFile`.
+        A path, never a value.
+      '';
+    };
+
     forgePublisherClientCertFile = lib.mkOption {
       type = lib.types.nullOr lib.types.str;
       default = null;
@@ -3524,6 +3571,8 @@ in
 
       systemd.services.swarm-bao-claude-login-policy = readerPolicyUnit "write the claude login minter's credentials-writer bao policy and cert-auth role" claudeLoginWriters;
 
+      systemd.services.swarm-bao-notify-router-policy = readerPolicyUnit "write the notification router's client-secret-reader bao policy and cert-auth role" notifyRouterReaders;
+
       systemd.services.swarm-bao-forge-publisher-policy = readerPolicyUnit "write the forge's controller-token-publisher bao policy and cert-auth role" forgePublisherWriters;
 
       # A FOURTH sibling, same shape and same reasons as the two above. This