swarm-nats-auth: verify an agent's own token against the store

An `auth_token` spelled `swarm-agent.<agent>.<secret>` is no longer sent
to introspection. The responder reads `swarm/agents/<agent>/queue` with
an identity of its own, checks that the stored object names the same
agent, compares the secret in constant time, and grants the subjects
`--agent-token-publish-subject` lists with `{agent}` expanded. Every
other outcome denies: a malformed token, no store identity, nothing
stored, a failed or slow lookup, a different secret. A token without
the prefix takes the OIDC path unchanged.

The journal's `auth request` line names such a caller `agent:<agent>`;
the hive-shared credential keeps `hive-<h>-agent`.

The new principal: a `swarm-nats-auth` cert-auth role and policy with
read on `secret/data/swarm/agents/+/queue` alone, a leaf signed by the
store's PKI glue, and `glue-nats-auth-bao-identity.nix` pairing the two.
The copy unit delivers the identity into the queue's container, and an
absent leaf is delivered empty so the responder still starts and only
agent tokens are refused.

The policy and role are written by `swarm-bao-nats-auth-policy`, logged in
as the bao granter: both names fall under its `swarm-*` globs, so the
deploy writes them with no operator step. module-eval counts it among the
granting units, so every generic granting-unit case covers it.

The secret compare uses `subtle`, already in the lock file through the
TLS stack; no workspace crate offered one directly.

nix/host-modules/swarm.nix

@@ -53,6 +53,7 @@ let
     deployCfg.bao.servicesIssuerCommonName
     deployCfg.bao.natsCommonName
     deployCfg.bao.granterCommonName
+    deployCfg.bao.natsAuthCommonName
   ]
   # The two per-hive readers' subjects, spelled out per hive rather than as the
   # prefix. The prefix alone would reserve the wrong string: the role for hive