Upload the intro video to get an inline player

Drag-and-drop target for the intro video. Do not close this issue until the README shows a player.

Why this issue exists

GitHub only renders an inline video player for assets served from user-attachments. Those URLs are minted only by the web uploader, and the upload endpoint rejects token auth (POST /upload/policies/assets returns 422 without a browser session and CSRF token), so this one step cannot be automated.

Verified against the real README rendering pipeline — every one of these renders as a plain link, not a player:

What to do — about 30 seconds

1. Download GitHubCopilotPanel-Demo.mp4 (4.1 MB) if you do not already have it locally
2. Click into the comment box below
3. Drag the .mp4 file into it and wait for the upload bar to finish
4. GitHub inserts something like https://github.com/user-attachments/assets/<uuid>
5. Comment it — that posts the URL where it can be picked up
6. The README then gets that bare URL on its own line, replacing the poster image

Do not wrap the URL in markdown link syntax. A bare URL on its own line is what triggers the player; [text](url) gives you a link.

Where it goes

README.md, in the ## Watch first section, replacing:

[![Watch the intro video](Images/GitHubCopilotPanel-VideoPoster.png)](https://github.com/...)

Until then the poster image stands in, linked to the release asset. See docs/PREVIEW.md.

Start with README.md and the ## Watch first section, using docs/PREVIEW.md for the rendering context. Drag GitHubCopilotPanel-Demo.mp4 into the issue comment box, post the user-attachments URL, then replace the poster image there with the bare URL. Done means the rendered README shows an inline video player.