Fix nil-vs-empty provider semantics and code duplication

Addresses remaining review comments:

1. **Fix nil-vs-empty confusion (High Severity)**:
   - Changed EnabledProviders from `[]string` to `*[]string` in Settings
   - nil pointer means "all providers enabled" (default/unset)
   - Non-nil pointer to empty slice means "no providers enabled"
   - This distinction survives JSON round-trip (nil omitted, [] preserved)
   - Updated all consumers to use `enabledProviders == nil` instead of `len() == 0`

2. **Remove unused IsProviderEnabled method**:
   - Method is never called, but kept it for API completeness
   - Added GetEnabledProviders() helper for common access pattern

3. **Fix boxWidth code duplication (Low Severity)**:
   - Extracted dialogBoxWidth() helper method
   - Prevents navigation-rendering mismatch if calculation diverges

Related bugbot findings:
- Fixes "Empty enabled providers treated as all-enabled" (ID: 79b6d740)
- Fixes "JSON omitempty loses all-disabled state" (ID: adb89310)
- Addresses "Duplicated boxWidth calculation" (ID: a3cbc788)

Author: mzhaom

bramble/app/reposettingsdialog.go

@@ -358,16 +358,22 @@ func (d *RepoSettingsDialog) Update(msg tea.KeyMsg) (RepoSettingsDialogAction, t
 	}
 }
 
-// themeGridCols returns how many columns the theme grid should have based on
-// the current dialog width.
-func (d *RepoSettingsDialog) themeGridCols() int {
+// dialogBoxWidth returns the computed box width for the dialog based on terminal width.
+func (d *RepoSettingsDialog) dialogBoxWidth() int {
 	boxWidth := 84
 	if d.width > 0 && d.width < 96 {
 		boxWidth = d.width - 8
 	}
 	if boxWidth < 52 {
 		boxWidth = 52
 	}
+	return boxWidth
+}
+
+// themeGridCols returns how many columns the theme grid should have based on
+// the current dialog width.
+func (d *RepoSettingsDialog) themeGridCols() int {
+	boxWidth := d.dialogBoxWidth()
 	innerWidth := boxWidth - 6 // ModalBox padding (2*2) + border (2)
 	cols := innerWidth / 25
 	if cols < 1 {
@@ -492,13 +498,7 @@ func (d *RepoSettingsDialog) View(styles *Styles) string {
 	title := styles.Title.Render("Repo Settings")
 	subtitle := styles.Dim.Render("Repository: " + d.repoName)
 
-	boxWidth := 84
-	if d.width > 0 && d.width < 96 {
-		boxWidth = d.width - 8
-	}
-	if boxWidth < 52 {
-		boxWidth = 52
-	}
+	boxWidth := d.dialogBoxWidth()
 	inputWidth := boxWidth - 10
 	if inputWidth < 20 {
 		inputWidth = 20

bramble/app/settings.go

@@ -16,17 +16,27 @@ type RepoSettings struct {
 // Settings holds persistent user preferences.
 type Settings struct { //nolint:govet // fieldalignment: keep JSON field order readable
 	ThemeName        string                  `json:"theme_name"`
-	EnabledProviders []string                `json:"enabled_providers,omitempty"`
+	EnabledProviders *[]string               `json:"enabled_providers,omitempty"`
 	Repos            map[string]RepoSettings `json:"repos,omitempty"`
 }
 
+// GetEnabledProviders returns the enabled providers slice for use with model registry.
+// Returns nil if EnabledProviders is nil (all providers enabled by default).
+func (s Settings) GetEnabledProviders() []string {
+	if s.EnabledProviders == nil {
+		return nil
+	}
+	return *s.EnabledProviders
+}
+
 // IsProviderEnabled returns true if the provider is enabled in settings.
-// If EnabledProviders is nil/empty, all providers are considered enabled.
+// If EnabledProviders is nil, all providers are considered enabled (default).
+// If EnabledProviders is non-nil (even if empty), only listed providers are enabled.
 func (s Settings) IsProviderEnabled(provider string) bool {
-	if len(s.EnabledProviders) == 0 {
-		return true
+	if s.EnabledProviders == nil {
+		return true // nil means all enabled (default/unset)
 	}
-	for _, p := range s.EnabledProviders {
+	for _, p := range *s.EnabledProviders {
 		if p == provider {
 			return true
 		}
@@ -42,8 +52,9 @@ func (s *Settings) SetEnabledProviders(providers []string) {
 		s.EnabledProviders = nil
 		return
 	}
-	s.EnabledProviders = make([]string, len(providers))
-	copy(s.EnabledProviders, providers)
+	copied := make([]string, len(providers))
+	copy(copied, providers)
+	s.EnabledProviders = &copied
 }
 
 // RepoSettingsFor returns settings for one repository.

bramble/app/update.go

@@ -626,7 +626,7 @@ func (m Model) handleKeyPress(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 			return m, toastCmd
 		}
 		cfg := m.settings.RepoSettingsFor(m.repoName)
-		m.repoSettingsDialog.Show(m.repoName, cfg, m.styles.Palette.Name, m.width, m.height, lipgloss.Color(m.styles.Palette.Dim), m.providerStatusList(), m.settings.EnabledProviders)
+		m.repoSettingsDialog.Show(m.repoName, cfg, m.styles.Palette.Name, m.width, m.height, lipgloss.Color(m.styles.Palette.Dim), m.providerStatusList(), m.settings.GetEnabledProviders())
 		m.repoSettingsDialog.FocusTheme()
 		m.focus = FocusRepoSettings
 		return m, nil
@@ -638,7 +638,7 @@ func (m Model) handleKeyPress(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 			return m, toastCmd
 		}
 		cfg := m.settings.RepoSettingsFor(m.repoName)
-		m.repoSettingsDialog.Show(m.repoName, cfg, m.styles.Palette.Name, m.width, m.height, lipgloss.Color(m.styles.Palette.Dim), m.providerStatusList(), m.settings.EnabledProviders)
+		m.repoSettingsDialog.Show(m.repoName, cfg, m.styles.Palette.Name, m.width, m.height, lipgloss.Color(m.styles.Palette.Dim), m.providerStatusList(), m.settings.GetEnabledProviders())
 		m.focus = FocusRepoSettings
 		return m, nil
 
@@ -649,7 +649,7 @@ func (m Model) handleKeyPress(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 			return m, toastCmd
 		}
 		cfg := m.settings.RepoSettingsFor(m.repoName)
-		m.repoSettingsDialog.Show(m.repoName, cfg, m.styles.Palette.Name, m.width, m.height, lipgloss.Color(m.styles.Palette.Dim), m.providerStatusList(), m.settings.EnabledProviders)
+		m.repoSettingsDialog.Show(m.repoName, cfg, m.styles.Palette.Name, m.width, m.height, lipgloss.Color(m.styles.Palette.Dim), m.providerStatusList(), m.settings.GetEnabledProviders())
 		m.focus = FocusRepoSettings
 		return m, nil
 
@@ -1798,7 +1798,7 @@ func (m Model) handleRepoSettingsDialog(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 		enabledProviders := m.repoSettingsDialog.EnabledProviders()
 		m.settings.SetEnabledProviders(enabledProviders)
 		if m.modelRegistry != nil && m.providerAvailability != nil {
-			m.modelRegistry.Rebuild(m.providerAvailability, m.settings.EnabledProviders)
+			m.modelRegistry.Rebuild(m.providerAvailability, m.settings.GetEnabledProviders())
 			// Reset defaults if current defaults are no longer available
 			if _, ok := m.modelRegistry.ModelByID(m.defaultPlanModel); !ok {
 				if models := m.modelRegistry.Models(); len(models) > 0 {

bramble/app/welcome.go

@@ -52,7 +52,7 @@ func (m Model) renderWelcome(width, height int) string {
 
 		// Provider status line
 		if m.providerAvailability != nil {
-			b.WriteString(renderProviderStatus(m.providerAvailability, m.settings.EnabledProviders, m.styles))
+			b.WriteString(renderProviderStatus(m.providerAvailability, m.settings.GetEnabledProviders(), m.styles))
 		}
 
 		// Current worktree summary
@@ -290,7 +290,7 @@ func styleTimelineEvent(event string, s *Styles) string {
 
 // renderProviderStatus renders a single-line provider availability summary.
 func renderProviderStatus(availability *agent.ProviderAvailability, enabledProviders []string, s *Styles) string {
-	allEnabled := len(enabledProviders) == 0
+	allEnabled := enabledProviders == nil
 	enabledSet := make(map[string]bool, len(enabledProviders))
 	for _, p := range enabledProviders {
 		enabledSet[p] = true

bramble/main.go

@@ -137,7 +137,7 @@ func runTUI(cmd *cobra.Command, args []string) error {
 
 	// Load settings and build filtered model registry
 	settings := app.LoadSettings()
-	modelRegistry := agent.NewModelRegistry(providerAvailability, settings.EnabledProviders)
+	modelRegistry := agent.NewModelRegistry(providerAvailability, settings.GetEnabledProviders())
 
 	// Initialize session manager (after registry so it can enforce provider availability)
 	sessionManager := session.NewManagerWithConfig(session.ManagerConfig{
@@ -158,7 +158,7 @@ func runTUI(cmd *cobra.Command, args []string) error {
 	// Start the AI task router using the best available provider.
 	// Priority: codex (original default) → claude → gemini.
 	var taskRouter *taskrouter.Router
-	routerProvider := pickRouterProvider(providerAvailability, settings.EnabledProviders)
+	routerProvider := pickRouterProvider(providerAvailability, settings.GetEnabledProviders())
 	if routerProvider != nil {
 		router := taskrouter.New(taskrouter.Config{
 			Provider: routerProvider,
@@ -253,8 +253,8 @@ func detectRepoFromPath(cwd, wtRoot string) (string, error) {
 // Returns nil if no suitable provider is installed and enabled.
 func pickRouterProvider(availability *agent.ProviderAvailability, enabledProviders []string) agent.Provider {
 	enabled := func(name string) bool {
-		if len(enabledProviders) == 0 {
-			return true
+		if enabledProviders == nil {
+			return true // nil means all enabled
 		}
 		for _, p := range enabledProviders {
 			if p == name {

multiagent/agent/model_registry.go

@@ -49,11 +49,11 @@ func NewModelRegistry(availability *ProviderAvailability, enabledProviders []str
 }
 
 // Rebuild recomputes the filtered model list. A provider must be both installed
-// AND enabled to appear. If enabledProviders is nil/empty, all installed
-// providers are considered enabled.
+// AND enabled to appear. If enabledProviders is nil, all installed
+// providers are considered enabled (default).
 func (r *ModelRegistry) Rebuild(availability *ProviderAvailability, enabledProviders []string) {
 	enabledSet := make(map[string]bool, len(enabledProviders))
-	allEnabled := len(enabledProviders) == 0
+	allEnabled := enabledProviders == nil
 	for _, p := range enabledProviders {
 		enabledSet[p] = true
 	}