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/renderer/components/PatternEditor.tsx

@@ -311,7 +311,7 @@ export function PatternEditor({
           </div>
         </div>
         <div className="no-drag flex items-center gap-2">
-          {!isBuiltin && onDelete && (
+          {onDelete && (
             <button
               className="flex items-center gap-1.5 rounded-lg px-3 py-1.5 text-[13px] text-[var(--color-status-error)] transition-all duration-200 hover:bg-[var(--color-status-error)]/10"
               onClick={onDelete}