Drop the last references to the removed hive matrix login form

`PutMatrixAccountRequest`'s no-`Debug` comment named the deleted
`MatrixLoginForm` as its precedent; it now states the reason directly.
The daemon's missing-sidecar warning told operators to re-login via the
dashboard, which no longer has that form; it now points at re-linking
the account from the swarm UI, whose stored credential carries the
homeserver the sidecar is written from. Behaviour is unchanged.

Refs #4348

hive-matrix-mcp/src/accounts.rs

@@ -208,7 +208,7 @@ fn discover_token_accounts_in(
                 account = name,
                 sidecar = %sidecar.display(),
                 "matrix: discovered token but no homeserver sidecar; skipping account \
-                 (re-login via the dashboard to write it)"
+                 (re-link the account from the swarm UI to write it)"
             );
             continue;
         };

swarm-controller/src/matrix_account.rs

@@ -81,9 +81,8 @@ pub(crate) fn configured_default_homeserver() -> Option<String> {
 
 /// The credential to store for one agent's external matrix account.
 ///
-/// No `Debug` derive — matching `hive-c0re::dashboard::matrix_accounts`'s
-/// analogous `MatrixLoginForm`, which also carries a password field and
-/// deliberately omits it so nothing can `{:?}`-log this by accident.
+/// No `Debug` derive: this carries a password and a token, so nothing may
+/// `{:?}`-log it by accident.
 #[derive(Deserialize, ToSchema)]
 pub struct PutMatrixAccountRequest {
     /// `"token"` (default, back-compat) or `"password"`. Token mode stores