swarmctl setup: an existing ruth keeps her permissions

declare_ruth stored the default set plus manage_root_agent over whatever ruth held, so a rerun dropped permissions granted since. It now reads her record and adds manage_root_agent only when it is missing, and writes nothing when she holds it. A new ruth still gets the defaults plus manage_root_agent.

swarmctl/src/permissions.rs

@@ -43,6 +43,18 @@ pub(crate) fn get(target: &Target, agent: &str) -> Result<()> {
     Ok(())
 }
 
+/// The stored record of `agent`, printing nothing.
+pub(crate) fn fetch(target: &Target, agent: &str) -> Result<Permissions> {
+    let uri = member(agent)?;
+    runtime()?.block_on(send(
+        target,
+        hyper::Method::GET,
+        &uri,
+        None::<&()>,
+        "permissions-get",
+    ))
+}
+
 /// Run `swarmctl agent permissions set`: store the given permissions, which
 /// replace the agent's record whole.
 pub(crate) fn set(