matrix: name the credential after the account it authenticates as

The store path and every identifier around it called this an admin
token. It is not one: of ~15 hive-c0re call sites only two need
homeserver admin, and the homeserver no longer promotes the account at
boot, so the name overstated both what the credential is and what it may
do.

Renaming it to the account was not enough either. "The `@hive:` token"
reads as the token of a hive user, and no such user is provisioned —
`@hive:<server_name>` is the appservice registration's own
`sender_localpart`, an account the homeserver creates for itself when it
loads the registration.

So it is the **sender token**: the matrix appservice sender account's
access token, at `swarm/services/matrix/sender-token`. The name says
what it authenticates as rather than what it may do, which is the part
that was wrong.

The path has one constructor, and the bao grant, the grant assertion and
three unit tests pin its literal independently — so a half-finished
rename fails a check rather than leaving the minter and its readers
disagreeing at runtime. `tracing` messages are renamed with the code, so
the journal reads the way the source does.

The host-side file keeps its name (`matrix/access-token`): it carried no
admin framing, and renaming it would orphan the file on every deployed
hive for nothing.

`docs/tools/hivectl-cli.md` is regenerated from the clap tree.

docs/getting-started/setup.md

@@ -229,7 +229,7 @@ control: [`swarm/ui.md`](../swarm/ui.md).
 ### 6 · Matrix
 
 ```bash
-# Ensure the hive's own `@hive:` account exists first
+# Ensure the appservice's sender account exists first
 hivectl matrix sync-admin
 
 # Provision ruth's own matrix account — same bootstrap-bypass reasoning
@@ -293,7 +293,7 @@ See [`tools/hivectl.md`](../tools/hivectl.md) for every `hivectl` verb.
 
 - **No forge admin token is stored in any agent state dir.** Agents
   hold a regular agent token in their `forge-token` file; sensitive
-  creds (the core token, the `@hive:` matrix access token) live on the host.
+  creds (the core token, the matrix sender token) live on the host.
 - All config changes (forge PRs on `agent-configs/<name>`) go through
   operator approval — agents can't unilaterally rebuild containers, by design.
   See [`boundary.md`](../trust-boundary/boundary.md) and [`security.md`](../trust-boundary/security.md).