Replace the committed README video with the GitDiagram video badge

GitDiagram hosts the one-minute tour and a badge for it, so the README
links there from the badge row and the mp4 and its poster leave the
repository. The package-contents test keeps guarding against media in
the npm tarball; its comment no longer names the deleted video.

Closes #190

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: D0n9X1n

README.md

@@ -3,9 +3,7 @@
 **Use Claude Code with the models available through your GitHub Copilot subscription.**
 
 [![CI](https://img.shields.io/github/actions/workflow/status/D0n9X1n/copilot-relay/ci.yml?branch=main&logo=githubactions&logoColor=white&label=CI)](https://github.com/D0n9X1n/copilot-relay/actions/workflows/ci.yml) [![npm version](https://img.shields.io/npm/v/copilot-relay?logo=npm)](https://www.npmjs.com/package/copilot-relay) [![npm downloads](https://img.shields.io/npm/dm/copilot-relay?logo=npm&label=downloads)](https://www.npmjs.com/package/copilot-relay) [![GitHub release](https://img.shields.io/github/v/release/D0n9X1n/copilot-relay?logo=github)](https://github.com/D0n9X1n/copilot-relay/releases/latest)
-[![Node.js](https://img.shields.io/badge/node-%3E%3D22-339933?logo=node.js&logoColor=white)](https://nodejs.org/) [![TypeScript](https://img.shields.io/badge/TypeScript-strict-3178C6?logo=typescript&logoColor=white)](tsconfig.json) [![Platforms](https://img.shields.io/badge/platform-macOS%20%7C%20Windows%20%7C%20Linux-lightgrey)](wiki/README.md) [![License: MIT](https://img.shields.io/github/license/D0n9X1n/copilot-relay?color=blue)](LICENSE)
-
-[![Watch the 80-second introduction to copilot-relay](https://raw.githubusercontent.com/D0n9X1n/copilot-relay/main/.github/assets/copilot-relay-explained.jpg)](https://github.com/D0n9X1n/copilot-relay/blob/main/.github/assets/copilot-relay-explained.mp4)
+[![Node.js](https://img.shields.io/badge/node-%3E%3D22-339933?logo=node.js&logoColor=white)](https://nodejs.org/) [![TypeScript](https://img.shields.io/badge/TypeScript-strict-3178C6?logo=typescript&logoColor=white)](tsconfig.json) [![Platforms](https://img.shields.io/badge/platform-macOS%20%7C%20Windows%20%7C%20Linux-lightgrey)](wiki/README.md) [![License: MIT](https://img.shields.io/github/license/D0n9X1n/copilot-relay?color=blue)](LICENSE) [![Watch a one-minute video tour of copilot-relay](https://gitdiagram.com/video-badge.svg)](https://gitdiagram.com/d0n9x1n/copilot-relay/video)
 
 ## Features
 

tests/unit/package-contents.test.ts

@@ -9,8 +9,8 @@ const execute = promisify(execFile)
 const root = fileURLToPath(new URL("../../", import.meta.url))
 
 // npm always packs package.json, README and LICENSE; the `files` allowlist in
-// package.json adds config.default.yaml and dist/. Repository media such as the
-// README's introduction video under .github/assets must never be published.
+// package.json adds config.default.yaml and dist/. Nothing else, and no image
+// or video file, may reach the published tarball.
 const allowedTopLevel = new Set(["package.json", "README.md", "LICENSE", "config.default.yaml"])
 
 test("npm package contains only the manifest, docs, default config and build output", { timeout: 60_000 }, async () => {