feat: allow deletion of built-in orchestration patterns

Remove the restriction that prevented users from deleting built-in
patterns (those with IDs prefixed 'pattern-'). Built-in patterns are
intended as starting points, not permanent fixtures.

Changes:
- Add deletedBuiltinPatternIds field to WorkspaceState to track which
  built-in patterns the user has removed
- Update mergePatterns to skip deleted built-ins during workspace load,
  preventing them from being re-added
- Remove the isBuiltinPattern guard from AryxAppService.deletePattern;
  when a built-in is deleted, its ID is recorded in the tracking list
- Enable the delete button in SettingsPanel and PatternEditor for all
  patterns (keep the 'Built-in pattern' label for context)
- Add 3 tests covering built-in deletion, custom deletion, and
  selectedPatternId fallback

Users can still restore deleted built-ins via workspace reset.

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

src/main/AryxAppService.ts

@@ -710,13 +710,15 @@ export class AryxAppService extends EventEmitter<AppServiceEvents> {
   }
 
   async deletePattern(patternId: string): Promise<WorkspaceState> {
-    if (isBuiltinPattern(patternId)) {
-      throw new Error('Built-in patterns cannot be deleted.');
-    }
-
     const workspace = await this.loadWorkspace();
     workspace.patterns = workspace.patterns.filter((pattern) => pattern.id !== patternId);
 
+    if (isBuiltinPattern(patternId)) {
+      const deletedIds = new Set(workspace.deletedBuiltinPatternIds ?? []);
+      deletedIds.add(patternId);
+      workspace.deletedBuiltinPatternIds = [...deletedIds];
+    }
+
     if (workspace.selectedPatternId === patternId) {
       workspace.selectedPatternId = workspace.patterns[0]?.id;
     }