feat: add native macOS desktop app

[danielkov]

Summary

• add a native SwiftUI macOS app with persisted workspaces, workspace conversations, new/resumed sessions, and one isolated kit serve process per open conversation
• mirror the TUI ACP experience with streaming messages and reasoning, tool/runtime cards, background compose controls, model and effort configuration, media attachments, context usage, cancellation, diagnostics, and Markdown rendering
• notify on completed background turns and highlight only unfocused conversations awaiting the user
• add robust ACP framing, request timeouts, process-group shutdown, CLI discovery/bundling, versioned atomic persistence, and config-default inheritance
• add an XcodeGen project, local build/run documentation, shared ACP fixture coverage, and bump Kit to 0.1.87