Starting a run on a machine without tmux surfaced `spawnSync tmux ENOENT`
as a 400 — blaming the request for a missing host package, naming no fix,
and reaching the user as a raw errno in the console pane.
The guard goes in spawnRun, which both spawn paths (POST /api/run and
POST /api/lanes/:id/start) already route through, so one check covers
them; the message carries the install command. Both routes map the code to
503: the request was fine, the dependency is not installed. The client
already renders `error.message` from the body verbatim, so no UI change.
Also: README's node advice said "stay on 20/22/24"; 1712 server + 488
client tests are green on v26.8.1, so only 25 is actually the bad one.
nntrivi2001
27313d4413