Add OpenAI Apps domain challenge file (#20677)

## Summary
- Add the OpenAI Apps domain verification token as a static well-known
file on the Twenty website.

## Why
The OpenAI Apps submission form allows a challenge base URL on the MCP
hostname or a parent hostname. Since the MCP hostname is
`api.twenty.com`, the parent origin `https://twenty.com` can serve the
challenge at `/.well-known/openai-apps-challenge` without adding an API
route.

## Validation
- `curl -I -L https://twenty.com/.well-known/openai-apps-challenge`
currently returns 404, confirming the file is not already live.
- `git diff --check origin/main...HEAD`
- Verified the PR diff is a single static file:
`packages/twenty-website-new/public/.well-known/openai-apps-challenge`.

## Submission setting
Use `https://twenty.com` as the Challenge Base URL after this is
deployed.

packages/twenty-website-new/public/.well-known/openai-apps-challenge

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+6-1tez_gHTN3rqJBhwcxsc6k1BM8vwglvmV7ERKHrSk