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

docs/tools/hivectl.md

@@ -50,31 +50,19 @@ Manual entry to the same idempotent matrix provisioning flow
 running (`services.hyperhive.deploy.matrix.enable = true`).
 
 ```bash
-hivectl matrix create-user mara         # create matrix account for a human; prints access_token to stdout
-hivectl matrix create-user mara --password hunter2  # set a client-login password
 hivectl matrix sync-admin               # provision / refresh the appservice's sender account
-hivectl matrix promote-user mara        # promote an existing matrix user to homeserver admin
-hivectl matrix reset-password iris      # generate and set a new random password for `iris`; prints it
 hivectl matrix invite mara              # invite a user to the hive Space
 hivectl matrix invite @mara:server --room '#hive-chat:server'  # ...or to a specific room/alias
 ```
 
-- `create-user`: for people and other non-agent accounts. It refuses
-  an agent's name: `swarm-controller` creates an agent's account and
-  stores its token where the agent's daemon reads it.
+Human matrix accounts come from SSO, not `hivectl`; matrix homeserver
+admin should eventually come from membership in authelia's `admins`
+group; nobody has built that sync yet.
+
 - `sync-admin`: ensures this hive's appservice sender account
   (`@hive-<hive>:<server_name>`, one per hive) exists
   (the account `hive-c0re` provisions rooms with). Token persisted to the
   access token path. Safe to run again — idempotent.
-- `promote-user`: promotes an already-registered user to homeserver
-  admin by an `!admin` command in `#admins`. ⚠️ Needs an admin **sender**,
-  which `@hive-<hive>:` isn't — promotion is a swarm-level operation, rehomed to
-  the swarm tier rather than granted here, so it has no admin sender to
-  call it with from the hive.
-- `reset-password`: asks the admin room to set a new random
-  password and prints it to stdout. ⚠️ Same admin-**sender** requirement,
-  and the same swarm-level rehoming, so it's unavailable from the hive
-  too. Useful if an agent or human lost credentials.
 - `invite`: invites a matrix user (full `@user:server` or a bare
   localpart, qualified with the homeserver's `server_name`) to the hive
   Space by default, or to a `--room` id / `#alias`. Uses the sender