[codex] improve Linux setup documentation

[Yazington]

What was wrong

The top-level documentation identifies Linux as supported, but it does not explain how the AppImage becomes a clickable desktop application or make the boundary between a desktop install, an npx server, and an enrolled execution machine explicit. FUSE, systemd, stale-service, and cleanup guidance is scattered or maintainer-focused, which makes a successful Add machine enrollment easy to mistake for a UI installation.

What changed

• Added an end-user Linux guide that starts with a setup-selection table.
• Documented x64 AppImage installation, distro-specific FUSE 2 packages, and a stable per-user application-menu launcher.
• Documented the requirements and lifecycle of npx bb-app and Add machine installations.
• Added systemd verification, 203/EXEC diagnosis, headless lingering guidance, browser-app guidance, and safe replacement/removal steps.
• Linked the guide from the root README, the bb-app package guide, and the desktop maintainer guide.

This is documentation-only; there are no protocol, runtime, or CLI changes.

How you verified

• corepack pnpm dlx prettier@3.8.3 --check README.md docs/linux.md packages/bb-app/README.md apps/desktop/README.md
• git diff --check
• Checked that all relative Markdown links in the four changed files resolve.
• Parsed the application-launcher shell block with bash -n.
• Confirmed libfuse2t64 is available on Ubuntu 24.04 and checked the other FUSE package names against their official distro package indexes.

Fixes

No linked issue.

AGENT GENERATED