swarm-nats: grant agents their hive's terminal subject

The responder has had an agent arm since the principal was minted, but
no deployment ever passed `--agent-publish-subject`, and an empty list
is a refusal by design: `Policy::permissions` returns `None` rather than
a grant that can do nothing, so every agent was turned away at CONNECT.
The subject itself is the one this thread settled on, `$SWARM.term`
namespaced per hive.

The value has to reach the responder with a literal dollar. systemd
substitutes `$NAME` in `ExecStart` whether or not the word is quoted,
so a single dollar expands `SWARM` — unset, therefore empty — and the
responder is handed `.term.{hive}.>`. That grant validates (it carries
the `{hive}` placeholder), is accepted, and matches nothing any agent
publishes to, so the failure surfaces as an authorization violation far
from its cause. `$$` in the unit text is the escape for one dollar.

The module-eval case reads the rendered unit rather than the module
source, because the single-dollar version renders perfectly well; the
doubled dollar is the only thing that distinguishes them before deploy.

The grant is per-hive, not per-agent: an agent's identity names its
hive, so any agent in a hive can publish as another. That is the
tradeoff ruled acceptable for now, tracked separately for tightening.

Refs #3805

nix/host-modules/swarm-nats.nix

@@ -783,6 +783,16 @@ in
                 "--hive-client-prefix ${lib.escapeShellArg autheliaCfg.hiveClientPrefix}"
                 "--agent-client-suffix ${lib.escapeShellArg autheliaCfg.agentClientSuffix}"
                 "--reader-client ${lib.escapeShellArg controllerCfg.queueClientId}"
+                # What an agent may publish to, `{hive}` standing for the
+                # hive its client id names. Without it the responder has no
+                # agent grant to hand out and refuses every agent at CONNECT,
+                # so the terminal stream below depends on this line existing.
+                #
+                # `$$`, not `$`: systemd substitutes `$NAME` in `ExecStart`
+                # whether or not it is quoted, so a single dollar reaches the
+                # responder as the empty expansion of an unset `SWARM` and the
+                # grant silently becomes `.term.{hive}.>`.
+                "--agent-publish-subject ${lib.escapeShellArg "\$\$SWARM.term.{hive}.>"}"
               ];
               # Every credential arrives by `LoadCredential` and is named
               # on the command line only as a **path** — `argv` is

nix/module-eval.nix

@@ -773,6 +773,24 @@ let
       name = "the queue's payload ceiling is set, not inherited from the server's default";
       ok = natsOldPath.containers.swarm-nats.config.services.nats.settings.max_payload == 8388608;
     }
+    {
+      # Reads the RENDERED unit text, not the module's source, because the
+      # failure this defends against renders perfectly: systemd substitutes
+      # `$NAME` in `ExecStart` regardless of quoting, so a single dollar
+      # here hands the responder `.term.{hive}.>` — a grant that parses, is
+      # accepted, and matches nothing an agent ever publishes to. Asserting
+      # the doubled dollar is the only way to tell the two apart before
+      # deploy. The flag's presence is asserted separately so that dropping
+      # the grant entirely fails as its own arm rather than as an escaping
+      # complaint.
+      name = "the responder grants agents their hive's terminal subject, and the dollar survives systemd";
+      ok =
+        let
+          exec =
+            natsOldPath.containers.swarm-nats.config.systemd.services.swarm-nats-auth.serviceConfig.ExecStart;
+        in
+        lib.hasInfix "--agent-publish-subject " exec && lib.hasInfix "$$SWARM.term.{hive}.>" exec;
+    }
     {
       # Reads the host's tmpfiles rules, not the options: the socket directory
       # nginx and the container share is created there, so a rename that