Add Appify UI 2026 document launcher for .webapp bundles

[subtleGradient]

Summary

• Introduce a new SwiftPM-based Appify UI 2026 app that opens .webapp document packages in a native AppKit/WebKit shell
• Define and validate a trusted webapp.json manifest format with subtleGradient-only, full-commit GitHub runner packages
• Add a deterministic app bundle build script, a Hello.webapp fixture, and a focused core test suite

Testing

• Added unit coverage for manifest parsing, trust validation, Bun resolution, runner argv construction, and open URL safety checks
• Verified the macOS app bundle build and ad-hoc code signing succeed
• Ran local smoke testing to confirm the built app launches as a real foreground macOS process with a menu bar and window