fix(run): refuse a run with ETMUXMISSING when the host has no tmux

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.

docs/API.md

@@ -245,6 +245,8 @@ All fields are optional. The tool (CLI invoked) comes from the lane itself, neve
 - **`approval`** — one of `on-request` or `never`, or omitted/empty for no approval prompt. Passed to `codex -a <value>`.
 - **`resumeSessionId`** or **`resumeLast: true`** — resume an earlier run instead of starting new. For Codex lanes specifically, `resumeLast: true` uses `codex resume --last`; `resumeSessionId` uses `codex resume <sessionId>`. For Claude lanes, `resumeSessionId` re-attaches a Claude session by id.
 
+Error `503 ETMUXMISSING` is returned when the host has no `tmux` binary. Every run the dashboard starts is a tmux session, so this is a missing dependency rather than a bad request — the message carries the install command, and the same code comes back from `POST /api/run`. Nothing else in the dashboard is affected.
+
 Error `409 ERUNLIVE` is returned if the lane already has a live run; stop it first with the `stop` action. The same code and status come back when the lane's tmux pane is already running an agent while a `prompt` or `resumeSessionId` was sent — that argv cannot be typed over a live agent, so the request is refused instead of answered 200 with nothing done. A start that asks for neither still adopts the existing pane silently.
 
 #### Preflight a destructive lane action