feat: add macOS menu bar app

[HexCodeYT]

Summary

Adds a native macOS menu bar shell for PlainLink. The app keeps PlainLink system-level: it controls the Rust CLI, LaunchAgent watcher, one-shot clipboard cleaning, restore, and diagnostics from the macOS status bar.

Details

• add plainlink clean-clipboard for one-shot current clipboard cleaning
• add Swift/AppKit PlainLink.app menu bar shell built with Apple Command Line Tools
• add app bundle metadata, build script, and smoke-test script
• add CI-safe app packaging tests
• update README, architecture diagrams, macOS docs, roadmap, and contributor notes
• remove browser/WASM framing from the core roadmap in favor of system clipboard/tray surfaces

Validation

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• scripts/test-macos-app.sh
• cargo run -- clean 'https://youtu.be/LYa_ReqRlcs?si=VC4qVB_EUC90uwbo'
• cargo run -- doctor

Notes

• Signing and notarization are intentionally out of scope for this PR.
• I did not run plainlink install or launch the UI in this environment because those mutate local LaunchAgent/UI state.