#22101 cmux-next: clear two base reds in cmux-next checks (Home shortcut open, cmux host dir)

Two guards in "cmux-next checks" fail on the feat-cmux-next tip (41cd51f8), so every PR inherits them:

• check-concurrency: HomeHostView.swift:159 (79b0ab2d, first-run Switching card) opens the Keyboard Shortcuts settings URL with a synchronous NSWorkspace.shared.open on the main thread. It now uses openInBackground, the sanctioned wrapper (cx-050n).
• check-state-roots: cmux-tui/src/remote_host/config.rs:74 (cmux host) defaults to <home>/.cmux/host. That is one listener config per machine, not a per-build root, and serve --dir and CMUX_HOST_DIR already override it. I added it as a reviewed keep row. @lawrencecchen, if the host should be per build, the row becomes a move.

Evidence: both scripts fail at 41cd51f8 and pass on this head. The third red on the tip, the error journal's try! (check-crash-safety), and the stale ci-target-graph.json are fixed in #22021.

Changelog: none

🤖 Generated with Claude Code

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Summary by cubic

Clears two failures in the cmux-next checks that redline every PR built on the feat-cmux-next tip.

• The Keyboard Shortcuts settings URL is now opened with openInBackground instead of a synchronous NSWorkspace.open on the main thread, satisfying check-concurrency.
• ~/.cmux/host is now a reviewed keep in the state-roots baseline: it is a machine-wide cmux host config (one listener per machine), not a per-build root, and serve --dir and CMUX_HOST_DIR already override it.