refactor(agent): drop vestigial install_configured param, non_empty_env helper, explicit infallible expect in client

hive-ag3nt/src/client.rs

@@ -76,7 +76,9 @@ where
             }
         }
     }
-    Err(last_err.unwrap_or_else(|| anyhow!("hive socket: retries exhausted")))
+    // Reaching here means the final attempt returned `Transient`, which always
+    // sets `last_err` — so this is infallible.
+    Err(last_err.expect("a transient failure on the final attempt set last_err"))
 }
 
 /// Transient = connect / IO error worth a retry (server restart, broken