fix: prefer witness agent override

internal/witness/manager_test.go

@@ -36,3 +36,20 @@ func TestBuildWitnessStartCommand_DefaultsToRuntime(t *testing.T) {
 		t.Errorf("expected BD_ACTOR=gastown/witness in command, got %q", got)
 	}
 }
+
+func TestBuildWitnessStartCommand_AgentOverrideWins(t *testing.T) {
+	roleConfig := &beads.RoleConfig{
+		StartCommand: "exec run --role {role}",
+	}
+
+	got, err := buildWitnessStartCommand("/town/rig", "gastown", "/town", "codex", roleConfig)
+	if err != nil {
+		t.Fatalf("buildWitnessStartCommand: %v", err)
+	}
+	if strings.Contains(got, "exec run") {
+		t.Fatalf("expected agent override to bypass role start_command, got %q", got)
+	}
+	if !strings.Contains(got, "GT_ROLE=witness") {
+		t.Errorf("expected GT_ROLE=witness in command, got %q", got)
+	}
+}