Add install.sh: automated macOS installer for Copilot for Xcode

[Copilot]

No streamlined CLI install path exists — users must know the Homebrew cask name or navigate GitHub releases manually.

install.sh (new)

• Validates macOS ≥ 12 and Xcode via xcode-select
• Installs via brew install --cask copilot-for-xcode when Homebrew is present; falls back to GitHub Releases API → ZIP download → extract → copy to /Applications
• Uses jq for JSON parsing when available, with grep/sed fallback
• Launches the app once to register its background launch agent
• Prints version-aware post-install steps (macOS 15 / 14 / older) for enabling the Source Editor Extension, granting Accessibility permission, and configuring an AI provider

bash <(curl -fsSL https://raw.githubusercontent.com/intitni/CopilotForXcode/main/install.sh)

README.md

• Install section leads with the one-liner above and a security note advising script review before execution
• Existing Homebrew and manual download instructions retained below

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