fix: drop --server flag from hivectl matrix; discover server_name from homeserver

Add matrix::discover_server_name() via GET /_matrix/key/v2/server
(unauthenticated federation endpoint, always returns server_name).
hivectl is always talking to the local hive — no reason to require
the operator to spell out the server_name.

hive-c0re/src/matrix.rs

@@ -598,6 +598,34 @@ pub async fn reset_user_password(
     )
 }
 
+/// Discover the matrix `server_name` from the running homeserver via
+/// `GET /_matrix/key/v2/server` (unauthenticated federation key endpoint).
+/// The response JSON always includes `"server_name"` per the matrix spec.
+pub async fn discover_server_name(client: &reqwest::Client) -> Result<String> {
+    let url = format!("{MATRIX_HTTP}/_matrix/key/v2/server");
+    let resp = client
+        .get(&url)
+        .send()
+        .await
+        .context("matrix: GET /_matrix/key/v2/server")?;
+    let status = resp.status();
+    let body = resp
+        .json::<serde_json::Value>()
+        .await
+        .context("matrix: parse /_matrix/key/v2/server response")?;
+    if !status.is_success() {
+        anyhow::bail!(
+            "matrix: /_matrix/key/v2/server returned HTTP {status}, body: {body}"
+        );
+    }
+    body["server_name"]
+        .as_str()
+        .map(str::to_owned)
+        .with_context(|| {
+            format!("matrix: /_matrix/key/v2/server response missing server_name field: {body}")
+        })
+}
+
 /// Read the hive admin access token from disk. Returns an error if it
 /// is absent — callers should gate their admin-API calls on this.
 pub fn read_admin_token() -> Result<String> {