fix(slurm): add default 30s timeout to HTTP client

Replaces http.DefaultClient with a client that has a 30s timeout to prevent indefinite hangs when the Slurm REST API is unresponsive.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/slurm/client_test.go

@@ -20,7 +20,7 @@ func TestNewClient(t *testing.T) {
 		t.Errorf("expected UserAgent %q, got %q", DefaultUserAgent, client.UserAgent)
 	}
 	if client.client == nil {
-		t.Error("expected http.Client to be initialized (nil httpClient should default to http.DefaultClient)")
+		t.Error("expected http.Client to be initialized (nil httpClient should create a client with default timeout)")
 	}
 
 	_, err = NewClient("://invalid", nil)