Add Cloud Agent development environment for the Server component

[kebkfdbbd]

Summary

Adds a repository-managed Cloud Agent environment (.cursor/environment.json) so this repo is usable in Cloud Agents.

Important scoping note

The primary product — GitHub Copilot for Xcode — is a macOS/Xcode app (platforms: [.macOS(.v13)], AppKit, XPC, Sparkle) that CI builds on macos-latest with xcodebuild. It cannot be built or run on a Linux Cloud Agent VM (no macOS SDK / Xcode). Therefore this environment is scoped to the only part of the repo that builds end-to-end on Linux: the Node/TypeScript Server/ component (the WebKit-hosted terminal and diffView UI bundles + language-server packaging).

What the environment does

install runs from /workspace:

cd Server && npm ci --force && npm run build

• --force is required because Server/package.json pins darwin-only @github/copilot-language-server-darwin-* binaries as hard dependencies; on Linux npm ci fails with EBADPLATFORM. --force installs the JS packages anyway. The webpack bundles (terminal, diffView) don't import those binaries, so the build is unaffected.
• Node 22 is already present in the default Cursor base image, so no custom Dockerfile/snapshot is needed.

Validation

• npm ci --force + npm run build succeed on the default base image (validated on this VM and in a fresh draft environment build).
• Install is deterministic (lockfile) and the build is idempotent (ran twice cleanly).
• Built terminal bundle renders an xterm.js terminal and window.writeToTerminal() works; diffView Monaco bundle loads with no console errors.