Expand Xcode onboarding docs and document core service APIs

[Copilot]

At the moment we are not accepting contributions to the repository.

This updates the repository’s Xcode-specific documentation surface and fills a few high-value API documentation gaps in the service layer. The main focus is faster setup, clearer account/auth flows, better feature discoverability, and more navigable core service code.

• README

  • Added build/actions/version badges
  • Added an Xcode-specific quick start
  • Clarified account requirements for Copilot sign-in vs. BYOK usage
  • Added a central documentation index for install, config, features, examples, troubleshooting, and contributing

• New docs

  • docs/INSTALLATION.md: Homebrew/DMG install flow, first-launch permissions, sign-in, updates
  • docs/CONFIGURATION.md: authentication, permissions, model selection, BYOK setup, recovery guidance
  • docs/FEATURES.md: Xcode integration capabilities, compatibility notes, and package/workspace references
  • docs/EXAMPLES.md: common Xcode scenarios for completions, chat, agent mode, reviews, and BYOK

• Contributor guidance

  • Added CONTRIBUTING.md
  • Updated expectations around Xcode workspace usage, target-level validation, docs upkeep, and secret handling

• Swift API documentation

  • Added focused doc comments to key orchestration types:
    • ServiceActor / Service
    • XPCService
    • ChatService
    • WorkspaceActor / WorkspacePool
    • XPCServiceProtocol
  • Kept code changes comment-only and limited to public/cross-module entry points

brew install --cask github-copilot-for-xcode
# Launch the app, grant required permissions, then sign in with a GitHub account that has Copilot access.