Add iPhone/iPad (iOS) support

[Copilot]

GitHub Copilot CLI had no iOS support — the install script would silently misidentify an iOS device as macOS and attempt to run an incompatible Darwin binary.

Changes

• install.sh: Detect iOS early in the Darwin branch via sysctl -n kern.ostype (returns "iOS" on iOS, "Darwin" on macOS). On iOS, exit with a clear message directing users to npm install -g @github/copilot instead of proceeding with an incompatible binary download.

  if [ "$(sysctl -n kern.ostype 2>/dev/null)" = "iOS" ]; then
    echo "iPhone/iPad (iOS) detected. The install script does not support iOS directly."
    echo "Please install via npm instead:"
    echo "  npm install -g @github/copilot"
    exit 1
  fi

• README.md: Added iPhone/iPad (iOS) to the supported platforms list; updated the npm install description to include iPhone; added a dedicated iPhone/iPad installation section referencing a-Shell as a compatible terminal app with a note that the curl/wget script path is unsupported on iOS.

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