generate-design Edit flow is a no-op — upstream edit_screens doesn't persist (see stitch-sdk#361)

The generate-design skill's Edit flow calls edit_screens, which currently returns success + dom_operations but never persists the change (the native Stitch web UI edit works; the programmatic path doesn't). Full repro + discriminator filed upstream at google-labs-code/stitch-sdk#361. Until the endpoint is fixed, the skill's "edit existing screen" path won't work for agent/SDK users — flagging here so people hitting it find the root cause.