docs/swarm/credentials: a hive is an identity too

Line 26's list named two identity kinds; mara flagged it as
should-be-three and the third went unnamed. A hive holds its own mTLS
leaf (glue-bao-tls.nix:130-131, CN = hiveName from :49) and logs in
with it (glue-queue-agent-credential.nix:171, bao login
-method=cert); swarm-bao.nix mints matching cert-auth roles for it
alongside swarm-controller (:1049-1052) and swarm-secret-publisher
(:1101-1104), and swarm-controller/src/read_policy.rs:184-188 writes
the per-hive role dynamically. The table further down this page
already lists a hive as a reader (swarm/hives/<hive>/matrix/appservice-token),
so the line-26 list was inconsistent with the rest of the page.

docs/swarm/credentials.md

@@ -23,9 +23,9 @@ the only credential on disk.** Each has to be a file, and the reason is the
 whole asymmetry: the certificate is what authenticates a principal to the
 store, so it's the one credential that can't be fetched from the store.
 Something has to exist on disk before the first request, or there's nothing
-to make the request with. Every identity — an agent, a swarm-level service —
-needs one; a host running several holds several, and its only power is to
-ask the store for the rest.
+to make the request with. Every identity — an agent, a hive, a swarm-level
+service — needs one; a host running several holds several, and its only
+power is to ask the store for the rest.
 `swarm-bao.nix:529-533` states the rule for the nix option that carries it:
 this is _"the credential an operator places by hand"_, and _"a path, never a
 value."_ A literal in a nix expression lands in the nix store —