fix(summer): pass delegated command flags to the app binary

- delegateCommand builds a Passthrough bonfire command, so
  `summer serve --addr :8123`, `summer serve --help` and `-h` reach the
  app binary instead of failing with "unknown flag: --addr"
- one leading "--" is dropped so `summer serve -- --addr :8123` still
  works
- appRunner is a package-level seam that tests replace to capture argv

cmd/summer/runtime_test.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"context"
+	"slices"
+	"testing"
+
+	"git.golem15.com/golem15/summercms/modules/bonfire"
+)
+
+// captureAppRunner replaces appRunner for the test and returns a pointer to
+// the argv of the last call.
+func captureAppRunner(t *testing.T) *[]string {
+	t.Helper()
+	got := new([]string)
+	prev := appRunner
+	appRunner = func(ctx context.Context, argv []string, out bonfire.Output) error {
+		*got = append([]string(nil), argv...)
+		return nil
+	}
+	t.Cleanup(func() { appRunner = prev })
+	return got
+}
+
+func TestDelegatedServePassesFlags(t *testing.T) {
+	for _, tc := range []struct {
+		args []string
+		want []string
+	}{
+		{[]string{"serve", "--addr", ":8123"}, []string{"serve", "--addr", ":8123"}},
+		{[]string{"serve", "--help"}, []string{"serve", "--help"}},
+		{[]string{"serve", "-h"}, []string{"serve", "-h"}},
+		{[]string{"serve", "--addr=:8123"}, []string{"serve", "--addr=:8123"}},
+		{[]string{"serve", "--", "--addr", ":8123"}, []string{"serve", "--addr", ":8123"}},
+	} {
+		got := captureAppRunner(t)
+		if out, err := runSummer(t, tc.args...); err != nil {
+			t.Fatalf("%q: %v\n%s", tc.args, err, out)
+		}
+		if !slices.Equal(*got, tc.want) {
+			t.Fatalf("%q: argv = %q, want %q", tc.args, *got, tc.want)
+		}
+	}
+}