Enroll demo.vercel.store in the WebMCP origin trial

WebMCP ships as a Chrome origin trial through Chrome 156. #1535 adds four WebMCP tools to the storefront and wires the token plumbing, but no origin-trial token is registered for demo.vercel.store.

Until one is, document.modelContext does not exist for ordinary visitors, so the tools register only for someone who has manually enabled chrome://flags/#enable-webmcp-testing. In other words, merging #1535 ships dormant code — the feature does not reach real Chrome users until this issue is done.

Steps

• Register https://demo.vercel.store for the WebMCP trial at developer.chrome.com/origintrials
• Set the issued token as WEBMCP_ORIGIN_TRIAL_TOKEN in the Vercel project for production
• Decide whether preview deployments get their own tokens or stay flag-only
• Note the Chrome 156 expiry somewhere durable, so the tools do not silently stop working when the trial ends

Notes

Tokens are origin-bound and expire. A token issued for demo.vercel.store does nothing on preview URLs, forks, or localhost, so each origin that needs the tools needs its own.

app/layout.tsx emits <meta http-equiv="origin-trial"> only when the environment variable is set, so leaving it unset is a clean no-op and there is no rush from a correctness standpoint — only from a "the feature is invisible" standpoint.

Filed separately from #1535 so it survives that PR being merged, squashed, or handed off.