matrix: mint the appservice sender token in the matrix container

A swarm runs one homeserver and a homeserver has one appservice sender
account, so "mint it once" is a property of the thing being minted
rather than something a lock has to enforce. That is what makes this
account the one to move first: no trigger route, no controller change
and no agent list — a boot-time oneshot beside tuwunel is the whole
mechanism.

`swarm-matrix-minter` runs inside `containers.hive-matrix`, which
already holds the appservice token: the rendered registration is bound
in read-only because that is how tuwunel is handed it. What the
container lacked was an identity of its own, so this adds one — a leaf
from the store's CA with a grant of exactly one path, not the hive's
leaf, which reads every secret in the store.

Both ends of the credential ship here. The minter reads the path it
publishes to before it touches the homeserver, and returning on a
non-empty read IS the "only once"; `hive-c0re`'s `ensure_hive_user`
reads the same path, authenticating with the hive name already in
`HYPERHIVE_HIVE_NAME`. The existing mint-then-`M_USER_IN_USE`-login
ladder stays as the fallback for a store that is empty, unconfigured or
unreachable, which is every swarm deployed before this — so nothing
needs backfilling and nothing breaks if the rest of the sequence never
lands.

The credential is not an admin credential, and is not named like one.
It is the access token of the appservice registration's own
`sender_localpart` — `@hive:<server_name>`, an account the homeserver
creates for itself when it loads the registration. The store path is
`swarm/services/matrix/sender-token`, the host path is
`matrix/access-token`, and the homeserver no longer runs an
`admin_execute` promotion for that account at boot. Everything the hive
provisions with it — the Space, the chat room, their hierarchy and join
rules, the invites — rides on being the creator of those rooms at power
level 100, not on homeserver admin; there is no Synapse admin API here
to need, tuwunel has none.

Two operations do need an admin *sender* and therefore stop working:
`hivectl matrix promote-user` and `hivectl matrix reset-password`, both
`!admin …` messages into `#admins:<server>`, plus the password-reset
recovery path that an agent with a lost password file falls back to.
They are swarm-level operations and are left failing loudly rather than
served by an over-privileged token every other call site would also
carry. The sweep's own admin-rights check and self-repair go with them:
an account that is deliberately not an admin has nothing to check.

`ephemeral = false` stays, and hive root can still read the container's
filesystem. Accepted: what this buys is identity separation — no hive
*process* holds or reads the appservice token — not physical isolation.

Refs #4345

nix/module-eval/bao-grants.nix

@@ -140,6 +140,56 @@ let
         baoGrantHere.systemd.services.swarm-bao-secret-publisher-policy.after
       );
     }
+    {
+      # The third principal's grant, and the narrowest of the three: ONE path,
+      # spelled to the leaf. The negative arms are the property — a homeserver
+      # is not entitled to overwrite Grafana's OIDC client, so widening this to
+      # the `services/` prefix the publisher holds would be a real loss even
+      # though it would read as tidier.
+      #
+      # ⚠️ `services` is PLURAL, because the path segment comes from
+      # `Kind::Service`'s `#[strum(serialize = "services")]` and not from
+      # `Kind::label`, which renders the singular for error text. The singular
+      # spelling evaluates, deploys, and 403s every read with "permission
+      # denied" and nothing else.
+      name = "the matrix minter's grant is the hive credential's path and nothing else";
+      ok =
+        let
+          s = baoGrantHere.systemd.services.swarm-bao-matrix-minter-policy.script;
+        in
+        lib.hasInfix "path \"secret/data/swarm/services/matrix/hive-access-token\" {" s
+        && !(lib.hasInfix "secret/data/swarm/services/*" s)
+        && !(lib.hasInfix "secret/data/swarm/agents" s)
+        && !(lib.hasInfix "secret/data/swarm/hives" s)
+        && !(lib.hasInfix "sys/policies/acl" s);
+    }
+    {
+      # 🩸 `read` is load-bearing here and is the one capability neither
+      # sibling has. The minter's first act is to read this path back and stop
+      # if something is there — that read IS "and only once", so without the
+      # capability every container restart would mint a second access token and
+      # invalidate the hive's.
+      name = "the matrix minter may read back the one path it writes";
+      ok =
+        let
+          s = baoGrantHere.systemd.services.swarm-bao-matrix-minter-policy.script;
+        in
+        lib.hasInfix "capabilities = [\"create\", \"update\", \"read\"]" s
+        && lib.hasInfix "auth/cert/certs/swarm-matrix-minter" s
+        && lib.hasInfix "allowed_common_names=swarm-matrix-minter" s;
+    }
+    {
+      # Same two controls its siblings carry: ordered after the unit that makes
+      # the mounts it writes into, and rendered on the HOST rather than inside
+      # the store's container, where it would have neither an identity nor a
+      # route to the store.
+      name = "the minter's granting unit is ordered after the mounts and rendered on the host";
+      ok =
+        lib.elem "swarm-bao-controller-policy.service" (
+          baoGrantHere.systemd.services.swarm-bao-matrix-minter-policy.after
+        )
+        && !(baoGrantHere.containers.swarm-bao.config.systemd.services ? swarm-bao-matrix-minter-policy);
+    }
     {
       # The policy authorising this route lives in another file, and nothing
       # else relates the grants to the paths the code actually writes.