test(cli): pin ExitCode mapping (usage=2, runtime=1, nil=0)

Closes the one nit from the regression audit: the exit-code wiring was correct
and covered end-to-end by the smoke test, but not pinned by a unit test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/internal/cli/exitcode_test.go

@@ -0,0 +1,27 @@
+package cli
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestExitCode(t *testing.T) {
+	cases := []struct {
+		name string
+		err  error
+		want int
+	}{
+		{"nil is success", nil, 0},
+		{"runtime error is 1", errors.New("boom"), 1},
+		{"usage error is 2", usageError{errors.New("bad flag")}, 2},
+		{"wrapped usage error is still 2", fmt.Errorf("ctx: %w", usageError{errors.New("x")}), 2},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := ExitCode(tc.err); got != tc.want {
+				t.Errorf("ExitCode(%v) = %d, want %d", tc.err, got, tc.want)
+			}
+		})
+	}
+}