feat: make global default timeout 0 (infinite) instead of required 30s

DefaultTimeoutSeconds now means "no timeout" when 0/empty, and that is
the new default, rather than an invalid config forcing a positive
value. runner.RunJob avoids context.WithTimeout with a zero duration
(which would expire immediately) and instead runs on a plain
cancelable context when no timeout is configured. Per-job
TimeoutSeconds inherit semantics are unchanged.

src/app/format_test.go

@@ -173,4 +173,7 @@ func TestDisplayTimeout(t *testing.T) {
 	if got, want := DisplayTimeout(inherit, 30), "30 s (global default)"; got != want {
 		t.Errorf("inherited timeout = %q, want %q", got, want)
 	}
+	if got, want := DisplayTimeout(inherit, 0), "no timeout (global default)"; got != want {
+		t.Errorf("inherited infinite timeout = %q, want %q", got, want)
+	}
 }