matrix, docs: drop the last admin framing, fix tense and prose lint

Three leftovers from the rename, plus the gating prose job.

`nix/packages/default.nix` still described the minter as the "matrix
admin credential's minter", and shipped that claim in the package's
`meta.description` — a PR-visible string.

`promote_user_to_admin`'s doc comment pointed at the `admin_execute`
entry in `hive-matrix.nix` as the thing that bootstraps the first
admin. That entry is gone, so the comment referenced nothing. It now
records that the account is ordinary, that the call therefore has no
working sender, and that rehoming at swarm level is the fix rather than
re-granting. `reset_user_password` gained the matching warning; it had
none.

The prose fixes clear all 8 `CI / prose lint (vale, errors)` failures,
all of which were in docs this branch touches. No vale config change,
no exception, no carve-out: contractions, one recast sentence, one
de-hyphenation and one dropped "simply".

Tense: four docs described system behaviour in the future ("will
refuse", "will fetch and trust"). Reference docs get read from the
other side of the change, so they say what the system does.

nix/packages/default.nix

@@ -163,12 +163,12 @@ in
   # rather than every hive's.
   swarm-nats-auth = mkBinPackage "swarm-nats-auth" "hyperhive swarm queue auth-callout responder";
 
-  # The matrix admin credential's minter. Out of `daemonBins` for the same
+  # The `@hive:` access token's minter. Out of `daemonBins` for the same
   # "runs *inside* a container, not on the host" reason as the responder
   # above, and with a second one: putting it in the core bundle would place
   # the binary that reads the appservice token on every hive's filesystem,
   # which is the arrangement it exists to end.
-  swarm-matrix-minter = mkBinPackage "swarm-matrix-minter" "hyperhive matrix admin-credential minter";
+  swarm-matrix-minter = mkBinPackage "swarm-matrix-minter" "hyperhive matrix @hive: access-token minter";
 
   # The only process allowed to write swarm-authelia's users database —
   # same "runs *inside* a container, not on the host" placement as