matrix: drop the per-hive appservice and its unused token

Every hive's homeserver loaded a per-hive `hyperhive` appservice
registration, and the swarm minted and delivered an as_token for it
through bao. No client presented that token: swarm-controller creates
every `@hive-<hive>` sender and every agent account with the swarm
appservice token (hive_sender.rs, agent_token.rs), and no Rust code read
the per-hive one.

Removed, with everything that existed only to serve it:
- hive-matrix.nix: the `hyperhive` registration, its activation-time
  render, the host bind mount of /var/lib/hyperhive/matrix-appservice,
  its LoadCredential entry, and the internal options
  `deploy.matrix.appserviceTokenFile` / `appserviceRegistrationScript`.
  tuwunel now loads only the swarm registration.
- glue-matrix-bao-token.nix and its `swarm-bao-matrix-token` unit.
- glue-bao-tls.nix: the matrix-token leaf and its defaults.
- swarm-bao.nix: the `swarm-matrix-token-<hive>` roles and policies
  (`swarm-bao-matrix-token-policy`), and the options
  `deploy.bao.matrixTokenClient{Cert,Key}File` and
  `matrixTokenCommonNamePrefix`, now mkRemovedOptionModule.
- swarm-secret-publisher.nix: the per-hive token mint and its state dir.
- swarm.nix: the reserved `<matrixTokenCommonNamePrefix>-<hive>` CN.
- swarm-secret-client: `matrix::appservice_token_path`.

The swarm registration (swarm-matrix-ctl, swarm-controller) is untouched.
Comments and docs that pointed at the removed glue file now point at
glue-queue-agent-credential.nix, which has the same reader shape.

Refs #4428

nix/host-modules/swarm-otel.nix

@@ -241,11 +241,10 @@ let
 
   # A reader of the store is defined by holding a certificate the store
   # accepts, never by standing next to it — the rule
-  # ./glue-matrix-bao-token.nix states in full. Unlike Grafana's identical-
+  # ./glue-queue-agent-credential.nix states in full. Unlike Grafana's identical-
   # looking flag, this one does not gate an assertion on its own: the
   # store-reading unit below simply does not render without it, the same choice
-  # ./glue-matrix-bao-token.nix and ./glue-queue-agent-credential.nix make for
-  # their own readers, because a collector with no client identity is
+  # ./glue-queue-agent-credential.nix makes for its own reader, because a collector with no client identity is
   # `haveCollectorSecret = false` above, and that is already a supported,
   # merely degraded shape rather than a service with no way in at all. What
   # IS asserted is narrower and lives in `assertions` below — see