feat(workflows): add phase 5 backend templates and export support

- add shared workflow template and serialization helpers for YAML, Mermaid, and DOT export
- add pattern-to-workflow upgrade and template application flows in AryxAppService
- persist built-in and custom workflow templates in workspace state and expose new IPC/preload methods
- cover the new backend slice with shared and app-service tests

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/shared/sessionLibrary.test.ts

@@ -77,6 +77,7 @@ function createWorkspace(overrides?: Partial<WorkspaceState>): WorkspaceState {
     projects: [createProject(), createProject({ id: 'project-scratchpad', name: 'Scratchpad', path: 'C:\\scratchpad' })],
     patterns: [createPattern(), createPattern({ id: 'pattern-single-chat', name: '1-on-1 Copilot Chat', mode: 'single' })],
     workflows: [],
+    workflowTemplates: [],
     settings: createWorkspaceSettings(),
     sessions: [
       createSession(),
@@ -117,6 +118,7 @@ function createWorkspace(overrides?: Partial<WorkspaceState>): WorkspaceState {
   return {
     ...workspace,
     workflows: overrides?.workflows ?? workspace.workflows,
+    workflowTemplates: overrides?.workflowTemplates ?? workspace.workflowTemplates,
   };
 }