hivectl, hive-c0re: remove dead matrix create-user/promote-user/reset-password

Human matrix accounts come from SSO, not hivectl. Matrix homeserver
admin will come from authelia's admins group (sync tracked in #4585);
password reset moves to swarm level (#4798). promote-user and
reset-password were already broken from the hive: the hive's sender
account has no admin sender to call the admin room with, only the
swarm's does.

Removes the three hivectl matrix verbs, their HostRequest variants,
their hive-c0re handlers, and the admin-room helpers (discover room id,
send-and-poll, event-id extraction, password/success parsing) that
only they used. sync-admin and invite are unchanged.

Refs #4585

README.md

@@ -111,17 +111,8 @@ For the full list of host and agent NixOS options see the
 
 `hivectl` is the operator-facing host CLI for ad-hoc administration that
 doesn't go through the broker (built alongside `hive-c0re` when the host
-module is enabled):
-
-```sh
-sudo hivectl matrix create-user mara                      # provisions a matrix user
-sudo hivectl matrix create-user mara --password-stdin     # … reading one line from stdin
-```
-
-For a name that's a managed agent, `hivectl` persists the resulting token
-to that agent's state dir, the same as the boot sweep does. For a
-non-agent name (for example the operator's own matrix account), it prints the
-token to stdout and writes nothing.
+module is enabled). Human matrix accounts come from SSO login, not
+`hivectl`.
 
 A human's first SSO login to the forge makes their forge account, not
 `hivectl`; `swarmctl forge make-admin <name>` on the swarm-controller's