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/plugin/plugin.go

@@ -68,6 +68,7 @@ type Contributions struct {
 	ActivityProviders  []ContributionActivityProvider `json:"activityProviders,omitempty"`
 	StatusBarItems     []ContributionStatusBarItem    `json:"statusBarItems,omitempty"`
 	OpenProviders      []ContributionOpenProvider     `json:"openProviders,omitempty"`
+	WorkspaceItems     []ContributionWorkspaceItem    `json:"workspaceItems,omitempty"`
 }
 
 // ContributionView represents a view contribution.
@@ -162,6 +163,14 @@ type ContributionOpenProvider struct {
 	Supports  []OpenProviderSupport `json:"supports"`
 }
 
+// ContributionWorkspaceItem represents a workspace tool contribution.
+type ContributionWorkspaceItem struct {
+	ID        string `json:"id"`
+	Title     string `json:"title"`
+	Icon      string `json:"icon,omitempty"`
+	Component string `json:"component"`
+}
+
 // SyncConfig describes plugin sync configuration.
 type SyncConfig struct {
 	Namespaces  []string `json:"namespaces,omitempty"`