[feat] add local iOS SPM package output

[adamTrz]

Summary

• add --add-spm-package to brownfield package:ios to generate a local Swift Package alongside the packaged XCFrameworks
• generate Package.swift and README.md for the local package, including Hermes, ReactBrownfield, Brownie, BrownfieldNavigation, React, and ReactNativeDependencies when present
• document how to integrate and verify the generated local package from a host iOS app

Why

We want an incremental path toward Swift Package Manager consumption of packaged RN iOS artifacts before the wider RN core migration lands.

Validation

• yarn workspace @callstack/brownfield-cli lint
• yarn workspace @callstack/brownfield-cli typecheck
• yarn workspace @callstack/brownfield-cli test
• manual validation: generated the local package from apps/RNApp, added it to AppleApp in Xcode, and confirmed the app runs without Metro

Notes

• host app project changes are intentionally excluded from this PR; AppleApp stays on the existing integration setup in-repo