feat: add a signed Desktop Tools menu bar app for agents outside Ace

[iamnbutler]

Part of #27. A standalone host so agents outside Ace (Claude Code or Codex through the MCP server in #12) get their own grants instead of borrowing Ace's (#24).

• native/sources/app/app.swift: a menu bar app (LSUIElement) that dlopens libDesktopTools.dylib like Ace does. It serves ~/.local/state/desktop-tools/desktop.sock for the exact client dev.githubnext.desktop-tools.desktop-client, links desktop-client there, and shows the runtime state and the three grants with grant buttons, plus a Serve agents toggle and Quit.
• src/app.ts:
  • assemble(out, scratch?) builds through dist/build.js, then lays out the bundle: executable, dylib, client, Licenses, Info.plist, and the swift-stdlib-tool Frameworks.
  • sign(app, identity, release) signs inside out, giving the client its exact identifier, adds the hardened runtime and timestamp for release, then runs codesign --verify --deep --strict.
  • Both also work as a CLI: node dist/app.js --out build --identity … --release.
  • feat: release a signed, notarized Desktop Tools DMG #28 uses both.
• build.ts also copies desktop-tools-app, a small AppKit-only target that doesn't link Peekaboo. Embedding apps can ignore it.
• The README documents the app and replaces "a vendor-signed companion … is not part of this package".

Verified locally:

• It builds with the local Developer ID identity (team 8S43JJMN6B).
• codesign shows the hardened runtime and timestamp, and the client identifier is right.
• spctl rejects it only as "Unnotarized Developer ID"; notarizing is feat: release a signed, notarized Desktop Tools DMG #28's job.
• From /Applications it starts serving and creates the 0600 socket and the client link.
• Live MCP runs follow once the grants are given.