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.

server/__tests__/run.test.js

@@ -118,6 +118,23 @@ describe("/api/run", () => {
     assert.equal(body.error.code, "EBADCWD");
   });
 
+  it("POST / answers 503, not 400, when the host has no tmux", async () => {
+    // The request is fine; the dependency is absent. 400 would tell the user to
+    // fix their input, and the body must carry the install line the UI shows.
+    tmux.__setExecImpl(() => {
+      const e = new Error("spawnSync tmux ENOENT");
+      e.code = "ENOENT";
+      throw e;
+    });
+    const { status, body } = await fetchJson("/api/run", {
+      method: "POST",
+      body: { laneId: 9, cwd: TMP },
+    });
+    assert.equal(status, 503);
+    assert.equal(body.error.code, "ETMUXMISSING");
+    assert.match(body.error.message, /install/i);
+  });
+
   it("POST / spawns a tmux session and GET /:id finds it", async () => {
     let hasSessionCalls = 0;
     tmux.__setExecImpl((args) => {