feat: add macos share extension

[rrbe]

Summary

• add a macOS Share Extension target that accepts Finder/system shared file URLs and forwards them to the existing app open handler
• package and sign the extension inside Contents/PlugIns with sandbox read-only entitlements
• document extension packaging, dependency boundaries, and local PluginKit testing notes

Tests

• swift build
• swift test
• ./build.sh
• temporary /Applications PluginKit registration smoke test