feat(cli): add --arm and populate run meta from it

Model and instructions are recorded only when the LLM picker is the one that
will actually run, so a spec declaring generator = llm() that is run under the
seeded picker does not label its trace with a model it never called.

Claude-Session: https://claude.ai/code/session_01A5KmftdEJ49A9z5mF5ESrX

cmd/sanderling/main.go

@@ -28,6 +28,7 @@ type testOptions struct {
 	androidAppPath string
 	duration       time.Duration
 	maxSteps       int
+	arm            string
 	seed           int64
 	output         string
 	clearData      bool
@@ -65,6 +66,7 @@ func parseTestArgs(args []string, stderr io.Writer) (testOptions, error) {
 	flagSet.Int64Var(&options.seed, "seed", 0, "RNG seed (0 = random)")
 	flagSet.StringVar(&options.output, "output", "./runs", "output directory for traces")
 	flagSet.BoolVar(&options.clearData, "clear-data", true, "clear app data before launching so each run starts from a fresh install; pass --clear-data=false to resume prior state")
+	flagSet.StringVar(&options.arm, "arm", "", "experiment cell label, recorded in meta.json so a directory of runs can be attributed to a cell")
 	flagSet.StringVar(&options.generator, "generator", "seeded", "action generator: seeded (weighted random) or llm (model picks from the same candidate set; requires generator = llm() in the spec)")
 	if err := flagSet.Parse(args); err != nil {
 		return testOptions{}, err

cmd/sanderling/main_test.go

@@ -297,3 +297,17 @@ func TestParseTestArgs_RejectsNegativeMaxSteps(t *testing.T) {
 		t.Fatal("expected an error for a negative --max-steps")
 	}
 }
+
+func TestParseTestArgs_ArmLabel(t *testing.T) {
+	options, err := parseTestArgs([]string{
+		"--spec", "s.ts",
+		"--bundle-id", "com.example",
+		"--arm", "seeded-identifier",
+	}, io.Discard)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if options.arm != "seeded-identifier" {
+		t.Errorf("arm: got %q, want seeded-identifier", options.arm)
+	}
+}

cmd/sanderling/test_run.go

@@ -23,5 +23,6 @@ func runTestPipeline(ctx context.Context, options testOptions, stdout io.Writer)
 		Output:         options.output,
 		ClearData:      options.clearData,
 		Generator:      options.generator,
+		Arm:            options.arm,
 	}, stdout)
 }