Build: Safari extension via safari-web-extension-converter

[paperhurts]

Spec'd in spec.md §'Build order' as 'Safari (1-2 days)' and tracked under #10.

Scope

• Convert the Chrome bundle via Apple's safari-web-extension-converter CLI.
• Xcode project lives in packages/extension-safari.
• Sign with an Apple Developer account ($99/yr) for stable distribution, or weekly free rebuild for personal use.

Out of scope

• Mac App Store listing.
• iOS Safari support (different bundle, separate plan).

Caveats

• Apple's converter and Xcode are macOS-only.
• Safari MV3 has divergences from Chrome MV3 (some chrome.* APIs are still in flux). The webextension-polyfill from the Firefox plan (Build: Firefox extension via webextension-polyfill #23) may need to share Safari abstractions too.

Acceptance

• A Safari extension target that loads via Settings → Extensions → For Developers, signs locally, and passes the same manual smoke test as the Chrome MVP.