Ignore dev shell files in the default worker template (#106)

## Summary

Ignore `.dev-shell/` in the default worker template so projects that
later add custom blocks do not track generated preview files. This
matches the existing custom block templates and covers generated
manifests, Vite configs, and caches.

## Test plan

- [x] `git check-ignore -v` confirms that generated manifests, run
configs, and caches match the template rule.
- [x] `git diff --check`

Author: jwenger-notion

workers/templates/workers-default/.gitignore

@@ -4,3 +4,4 @@ dist/
 node_modules/
 workers.*.json
 workers.json
+.dev-shell/