AppImage packaging for the cluster worker

[jaylfc]

Spec: `docs/superpowers/specs/2026-04-14-worker-appimage-design.md`.

Current worker install is a bash/powershell script that clones the repo and runs pip. An AppImage would mean:

• One download, runs on any glibc Linux ≥ 2.28
• No Python version surprises (bundled interpreter)
• Desktop file + icon for non-technical users
• Self-update via AppImageUpdate

Build pipeline: GitHub Actions matrix (x86_64, aarch64) → linuxdeploy → sign → release asset.