perf(ci): share UI artifact cache across runner OSes

The cached HTML output is platform-independent, so set
enableCrossOsArchive on the cache step. With this any OS can restore
the cache populated by any other OS — one shared cache instead of three.

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

Author: SamMorrowDrums

.github/actions/build-ui/action.yml

@@ -13,6 +13,7 @@ runs:
           pkg/github/ui_dist/issue-write.html
           pkg/github/ui_dist/pr-write.html
         key: ui-dist-v1-${{ hashFiles('ui/package-lock.json', 'ui/package.json', 'ui/index.html', 'ui/tsconfig*.json', 'ui/vite.config.ts', 'ui/src/**', 'ui/scripts/**') }}
+        enableCrossOsArchive: true
 
     - name: Set up Node.js
       if: steps.cache-ui.outputs.cache-hit != 'true'