feat: milestones 6b-fix through 6e — default-editor, files plugin, workspace host, workspaceItems contribution

- Fix PluginCard openProviders display
- Add default-editor plugin (text/markdown/notes-context)
- Add files plugin with workspaceItems placement
- Add workspaceItems contribution point (Go + API + mock + SDK)
- Add WorkspaceHost component for workspace area
- WorkspaceTree dispatches selection event
- Fix default-editor layout to fill container
- Fix PluginCard unsafe .length access
- Add E2E tests: 34/34 pass
- Add bundle execution check to official-plugins check.sh
- Update docs: PLUGIN_RUNTIME, DEV_PLUGINS, MILESTONE_6B/6C/6D plans

internal/core/contribution/registry.go

@@ -23,6 +23,7 @@ type Registry struct {
 	activityProviders []ContributionActivityProvider
 	statusBarItems    []ContributionStatusBarItem
 	openProviders     []ContributionOpenProvider
+	workspaceItems    []ContributionWorkspaceItem
 }
 
 // ContributionPointType defines the type of contribution point.
@@ -40,6 +41,7 @@ const (
 	PointActivity        ContributionPointType = "activityProviders"
 	PointStatusBar       ContributionPointType = "statusBarItems"
 	PointOpenProviders   ContributionPointType = "openProviders"
+	PointWorkspaceItems  ContributionPointType = "workspaceItems"
 )
 
 // ListByPoint returns all contributions for a given point type.
@@ -93,6 +95,10 @@ func (r *Registry) ListByPoint(point ContributionPointType) []interface{} {
 		for _, v := range r.openProviders {
 			result = append(result, v)
 		}
+	case PointWorkspaceItems:
+		for _, v := range r.workspaceItems {
+			result = append(result, v)
+		}
 	}
 	return result
 }
@@ -147,6 +153,12 @@ type ContributionOpenProvider struct {
 	Item     plugin.ContributionOpenProvider `json:"item"`
 }
 
+// ContributionWorkspaceItem is a workspace tool contribution.
+type ContributionWorkspaceItem struct {
+	PluginID string                           `json:"pluginId"`
+	Item     plugin.ContributionWorkspaceItem `json:"item"`
+}
+
 // NewRegistry creates a new contribution registry.
 func NewRegistry() *Registry {
 	return &Registry{}
@@ -171,6 +183,7 @@ func (r *Registry) Register(pluginID string, c *plugin.Contributions) {
 	r.activityProviders = removeActivityProviders(r.activityProviders, pluginID)
 	r.statusBarItems = removeStatusBarItems(r.statusBarItems, pluginID)
 	r.openProviders = removeOpenProviders(r.openProviders, pluginID)
+	r.workspaceItems = removeWorkspaceItems(r.workspaceItems, pluginID)
 
 	for _, item := range c.Views {
 		r.views = append(r.views, ContributionView{PluginID: pluginID, Item: item})
@@ -205,6 +218,9 @@ func (r *Registry) Register(pluginID string, c *plugin.Contributions) {
 	for _, item := range c.OpenProviders {
 		r.openProviders = append(r.openProviders, ContributionOpenProvider{PluginID: pluginID, Item: item})
 	}
+	for _, item := range c.WorkspaceItems {
+		r.workspaceItems = append(r.workspaceItems, ContributionWorkspaceItem{PluginID: pluginID, Item: item})
+	}
 }
 
 // Unregister removes all contributions from a plugin.
@@ -223,6 +239,7 @@ func (r *Registry) Unregister(pluginID string) {
 	r.activityProviders = removeActivityProviders(r.activityProviders, pluginID)
 	r.statusBarItems = removeStatusBarItems(r.statusBarItems, pluginID)
 	r.openProviders = removeOpenProviders(r.openProviders, pluginID)
+	r.workspaceItems = removeWorkspaceItems(r.workspaceItems, pluginID)
 }
 
 // Getters — sorted for deterministic display.
@@ -304,6 +321,20 @@ func (r *Registry) OpenProviders() []ContributionOpenProvider {
 	return result
 }
 
+func (r *Registry) WorkspaceItems() []ContributionWorkspaceItem {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	result := make([]ContributionWorkspaceItem, len(r.workspaceItems))
+	copy(result, r.workspaceItems)
+	sort.Slice(result, func(i, j int) bool {
+		if result[i].PluginID != result[j].PluginID {
+			return result[i].PluginID < result[j].PluginID
+		}
+		return result[i].Item.ID < result[j].Item.ID
+	})
+	return result
+}
+
 // ─── Remove helpers ─────────────────────────────────────────
 
 func removeViews(items []ContributionView, pluginID string) []ContributionView {
@@ -405,3 +436,13 @@ func removeOpenProviders(items []ContributionOpenProvider, pluginID string) []Co
 	}
 	return result
 }
+
+func removeWorkspaceItems(items []ContributionWorkspaceItem, pluginID string) []ContributionWorkspaceItem {
+	var result []ContributionWorkspaceItem
+	for _, item := range items {
+		if item.PluginID != pluginID {
+			result = append(result, item)
+		}
+	}
+	return result
+}