feat(#2035): auto-discover dashboard-provisioned matrix accounts via token+homeserver sidecar

hive-sh4re/src/priv_proto.rs

@@ -389,6 +389,13 @@ pub enum PrivRequest {
         /// Extra-account suffix. `None` → `matrix-token` (the hive account);
         /// `Some(name)` → `matrix-token-<name>` (validated as a plain ident).
         account: Option<String>,
+        /// Homeserver URL for an extra account. When `Some` (only meaningful
+        /// alongside `account: Some`), hive-priv also writes the sidecar
+        /// `matrix-account-<name>.json` (`{"homeserver": <url>}`, 0600,
+        /// chowned to the agent) so the daemon can auto-discover the account
+        /// without a config declaration. `None` → no sidecar written.
+        #[serde(default)]
+        homeserver: Option<String>,
     },
 
     /// Restart `hive-matrix-daemon.service` inside an agent container via