hive-c0re: stop minting agents' matrix accounts

The swarm mints each agent's `main` account now, so the hive's own mint
goes: `ensure_user_for`, `finish_user_provisioning`, `sync_agent`,
`sync_agent_standalone`, `token_path`, `legacy_password_path`,
`auto_reset_password` and `token_file_present`, and the calls from the startup sweep and the
rebuild bookkeeping. Both mints pinned the device `hyperhive-<agent>`, so
leaving this one would have each re-login kill the other's token.

`hivectl matrix create-user` refuses an agent's name and says where its
account comes from. Everything that still uses the hive's appservice token
stays: the hive's own account, the Space and chat room, and operator
accounts.

docs/tools/hivectl.md

@@ -50,7 +50,6 @@ Manual entry to the same idempotent matrix provisioning flow
 running (`services.hyperhive.deploy.matrix.enable = true`).
 
 ```bash
-hivectl matrix create-user iris         # provision (or re-provision) matrix account for agent `iris`
 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
@@ -60,9 +59,9 @@ 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 agents, persists the `access_token` to
-  `<state>/matrix-token`. Skips registration when the file already
-  exists — delete it first to force re-registration.
+- `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.
 - `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