matrix: one sender account and one sender token per hive

A swarm runs one homeserver and every hive on it logged in as the same
`@hive:` localpart, holding the same access token out of one swarm-wide
store path. That is one matrix identity for N hives: the homeserver
cannot attribute an action to the hive that took it, and revoking one
hive's standing revokes every hive's.

Three changes, and the third is the one that makes the other two real:

- **The localpart carries the hive's name** (`hive-<hive>`), derived in
  one place, `swarm_secret_client::matrix::hive_localpart`.
  `hive-matrix.nix` renders the same string as the appservice
  registration's `sender_localpart`, so the shared account stops being
  created rather than merely stops being used.
- **The store path is templated by hive**, not a constant. The
  "a swarm runs one homeserver, so this is a constant rather than a
  parameter" rationale went with it; it stopped holding the moment two
  hives shared the homeserver it describes.
- **The path moved out from under the grant every hive has.** It sat at
  `swarm/services/matrix/sender-token`, inside the
  `secret/data/swarm/services/*` read stanza `policy::render` gives every
  hive. It now sits under that hive's own stanza,
  `secret/data/swarm/hives/<hive>/*`, which interpolates the reader's
  name — so a hive reads its own token and is refused another's. The
  policy renderer itself is unchanged: narrowing the `services/*` grant
  would break the OIDC-secret read it exists for, and moving the
  credential is what this needed instead. A policy test walks the
  rendered stanzas and asserts none of hive alpha's covers hive beta's
  sender token, so a later stanza that widened it fails here.

`swarm-matrix-ctl` takes a new required `MATRIX_MINT_HIVE` and writes
that hive's path; its store grant in `swarm-bao.nix` follows, scoped to
one hive's leaf via the new `deploy.bao.matrixCtlHiveName` (defaulting to
this host's `hiveName`) rather than a `hives/*` wildcard, which would
hand the matrix container every hive's token back.

Migration: no outage at deploy. `ensure_hive_user` short-circuits on the
local token file, so a hive keeps running on what it has; with no such
file it reads the new per-hive path, finds nothing, and falls through to
the existing register-or-appservice-login ladder against its own
localpart — which needs only the per-hive `as_token` on local disk. The
old shared object is read by nothing afterwards. Rooms do not follow the
identity, and that is the one operator step; both ways out are written
into `docs/integrations/matrix.md`.

No admin standing is granted to the per-hive accounts: `admin_execute`
stays empty and the assertion pinning it is untouched.

nix/host-modules/hive-matrix.nix

@@ -73,10 +73,21 @@ let
   # anything, so the account exists on the very first boot of a fresh
   # homeserver without hive-c0re racing to register it.
   #
-  # ⚠️ Must equal `matrix::HIVE_LOCALPART` in hive-c0re, which derives
-  # it independently with nothing wiring an override across — same
-  # agreement, and same reason for saying so, as the token path below.
-  hiveLocalpart = "hive";
+  # 🩸 **Derived from the hive name, so it is one account per hive.** It was
+  # the bare `hive` and a swarm runs one homeserver: every hive on it acted
+  # as the same `@hive:`, which is one identity with no attribution and no
+  # way to revoke a single hive. The hives that do not run this container
+  # register their own `hive-<name>` through the appservice namespace below,
+  # which already admits it.
+  #
+  # ⚠️ Must equal `swarm_secret_client::matrix::hive_localpart`, which
+  # hive-c0re and swarm-matrix-ctl both derive from independently with
+  # nothing wiring an override across — same agreement, and same reason for
+  # saying so, as the token path below.
+  #
+  # `hiveName` has no fallback here for the reason ./glue-matrix-bao-token.nix
+  # gives at its own use of it: it is asserted set for every hyperhive host.
+  hiveLocalpart = "hive-${toString config.services.hyperhive.hiveName}";
 
   # The `as_token`, and the `hs_token` the spec requires alongside it. Both
   # minted by the render script below, mode 0600; the `as_token` is the one
@@ -168,7 +179,7 @@ let
   # knows about itself.
   ctlHomeserverUrl = if cfg.gatewayHost == null then "" else "https://${toString cfg.gatewayHost}";
 
-  # Every local user this hive may provision — agents, `@hive:` itself, and
+  # Every local user this hive may provision — agents, `@hive-<hive>:` itself, and
   # the operator accounts `hivectl matrix create-user` makes, which is the
   # whole matrix localpart charset.
   #
@@ -1439,6 +1450,12 @@ in
               # never a value.
               MATRIX_MINT_REGISTRATION = appserviceRegistrationPath;
               MATRIX_MINT_LOCALPART = hiveLocalpart;
+              # The hive segment of the store path the token is published
+              # under, and so the thing that keeps this hive's token out of
+              # every other hive's reach: the grant that reaches it is the
+              # hive's own `swarm/hives/<name>/*` stanza. ./swarm-bao.nix
+              # spells the same name into matrix-ctl's write grant.
+              MATRIX_MINT_HIVE = toString config.services.hyperhive.hiveName;
               MATRIX_MINT_HOMESERVER = ctlHomeserverUrl;
             }
             // lib.optionalAttrs (deployCfg.bao.serverCaFile != null) {