docs: document macOS-only build constraints for Cloud Agents

[WinfredGe]

Summary

Attempted to set up the development environment for PopBar in the Cursor Cloud Agent VM (Ubuntu 24.04 x86_64).

PopBar is a native macOS 15+ menu-bar GUI app (pure Swift/AppKit, no third-party deps, built via swiftc in build.sh). It cannot be built, run, or functionally tested on the Linux Cloud Agent VM — this is a platform incompatibility, not a fixable environment issue.

Evidence

Installed Swift 6.3.3 for Linux as a remediation attempt and compiled the sources:

• Foundation-only files (HTMLToMarkdown.swift, SelectionPayload.swift) → compile OK.
• import Cocoa (12 of 14 files) → error: no such module 'Cocoa' (Apple frameworks absent on Linux).
• Real build target arm64-apple-macosx15.0 → unable to load standard library for target 'arm64-apple-macosx15.0' (no macOS SDK).

It is also a GUI app (LSUIElement) requiring a macOS window server + Accessibility (TCC) permission.

Changes

• Adds AGENTS.md with a ## Cursor Cloud specific instructions section documenting:
  • the macOS-only platform requirement and why the app cannot run on Linux,
  • the real build/run path (./build.sh on macOS, see README.md),
  • that there are no automated tests / lint config / CI in the repo,
  • the two Foundation-only files that can be type-checked on Linux.

Update script

Set to a no-op (true) — there are no cross-platform dependencies to refresh on the Linux VM, and installing the ~1 GB Linux Swift toolchain on every startup is not worth it for two Foundation-only files (reliability-critical infra should stay minimal).