fix(formula): set rigPath when falling back to gastown default

When `gt formula run` fell back to the default "gastown" rig (because no
rig could be detected), it didn't set rigPath, which meant the default
formula lookup would fail. Now rigPath is properly constructed when we
have townRoot but can't detect a current rig.

Also adds tests for GetDefaultFormula helper.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/loader_test.go

@@ -1577,6 +1577,44 @@ func TestSaveTownSettings(t *testing.T) {
 	})
 }
 
+func TestGetDefaultFormula(t *testing.T) {
+	t.Run("returns empty string for nonexistent rig", func(t *testing.T) {
+		result := GetDefaultFormula("/nonexistent/path")
+		if result != "" {
+			t.Errorf("GetDefaultFormula() = %q, want empty string", result)
+		}
+	})
+
+	t.Run("returns empty string when no workflow config", func(t *testing.T) {
+		dir := t.TempDir()
+		settings := NewRigSettings()
+		if err := SaveRigSettings(RigSettingsPath(dir), settings); err != nil {
+			t.Fatalf("SaveRigSettings: %v", err)
+		}
+
+		result := GetDefaultFormula(dir)
+		if result != "" {
+			t.Errorf("GetDefaultFormula() = %q, want empty string", result)
+		}
+	})
+
+	t.Run("returns default formula when configured", func(t *testing.T) {
+		dir := t.TempDir()
+		settings := NewRigSettings()
+		settings.Workflow = &WorkflowConfig{
+			DefaultFormula: "shiny",
+		}
+		if err := SaveRigSettings(RigSettingsPath(dir), settings); err != nil {
+			t.Fatalf("SaveRigSettings: %v", err)
+		}
+
+		result := GetDefaultFormula(dir)
+		if result != "shiny" {
+			t.Errorf("GetDefaultFormula() = %q, want %q", result, "shiny")
+		}
+	})
+}
+
 // TestLookupAgentConfigWithRigSettings verifies that lookupAgentConfig checks
 // rig-level agents first, then town-level agents, then built-ins.
 func TestLookupAgentConfigWithRigSettings(t *testing.T) {