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__/pty-run.test.js

@@ -58,6 +58,29 @@ describe("pty-run", () => {
     assert.ok(newSessionCall.includes("opus"));
   });
 
+  it("spawnRun refuses with ETMUXMISSING, naming the fix, when tmux is not installed", () => {
+    // Before this guard the ENOENT escaped from three calls down as
+    // `spawnSync tmux ENOENT` and the route answered 400 — blaming the request
+    // for a missing host package, with no hint of what to install.
+    const calls = [];
+    tmux.__setExecImpl((args) => {
+      calls.push(args);
+      const e = new Error("spawnSync tmux ENOENT");
+      e.code = "ENOENT";
+      throw e;
+    });
+
+    assert.throws(
+      () => pty.spawnRun({ laneId: 42, cwd: "/tmp/repo" }),
+      (err) => err.code === "ETMUXMISSING" && /install/i.test(err.message),
+      "a missing tmux must be reported as such, with the install command"
+    );
+    assert.ok(
+      !calls.some((c) => c[0] === "new-session"),
+      "must not attempt a session it already knows cannot be created"
+    );
+  });
+
   it("spawnRun is a no-op (adopts) when the existing session's pane runs claude", () => {
     const calls = [];
     tmux.__setExecImpl((args) => {