fix(#2560): kill_on_drop for git + nix children so timeouts don't orphan

hive-c0re/src/meta.rs

@@ -1391,6 +1391,10 @@ async fn nix_output(dir: &Path, args: &[&str]) -> Result<std::process::Output> {
     Command::new("nix")
         .current_dir(dir)
         .args(nix_argv(args))
+        // Kill the nix child if the caller's future is dropped (e.g. the
+        // startup-migration timeout around `sync_agents` fires) rather than
+        // orphaning it against an unreachable forge. No-op on normal exit.
+        .kill_on_drop(true)
         .output()
         .await
         .with_context(|| format!("nix {} in {}", args.join(" "), dir.display()))