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) => {

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) => {

server/lib/pty-run.js

@@ -181,6 +181,21 @@ function spawnRun(args) {
     throw makeErr("EBADSESSION", "resumeSessionId is not a valid session id");
   }
 
+  // A run IS a tmux session, so an absent binary is a precondition failure, not
+  // a spawn failure. Without this the caller gets `spawnSync tmux ENOENT` from
+  // three directories down — a message that names no fix — and the route maps
+  // it to 400, blaming the request for the host's missing package. Checked here
+  // rather than in each route because both spawn paths (POST /api/run and
+  // POST /api/lanes/:id/start) come through this function.
+  if (!tmux.isTmuxAvailable()) {
+    throw makeErr(
+      "ETMUXMISSING",
+      "tmux is not installed, and every run the dashboard starts is a tmux session. " +
+        "Install it (sudo apt install tmux / brew install tmux; on Windows run AgentHub under WSL2), " +
+        "then restart the dashboard. Everything else keeps working without it."
+    );
+  }
+
   const id = runIdForLane(laneId);
   const startedAt = Date.now();
   const argv = buildArgv({

server/routes/lanes.js

@@ -1360,6 +1360,12 @@ router.post("/:id/:action", sameOriginGuard, async (req, res) => {
         break;
     }
   } catch (err) {
+    // A host with no tmux cannot start a run, but nothing is broken on this
+    // side — 503 plus the install line, not a 500 the reader can only read as a
+    // dashboard bug.
+    if (err.code === "ETMUXMISSING") {
+      return res.status(503).json({ error: { code: err.code, message: err.message } });
+    }
     return res.status(500).json({ error: { code: err.code, message: err.message } });
   }
 

server/routes/run.js

@@ -260,6 +260,11 @@ router.post("/", (req, res) => {
     if (err.code === "ERUNLIVE") {
       return res.status(409).json({ error: { code: err.code, message: err.message } });
     }
+    // Nor is a host missing tmux: the request was fine, the dependency is not
+    // installed. 400 would tell the user to fix their input.
+    if (err.code === "ETMUXMISSING") {
+      return res.status(503).json({ error: { code: err.code, message: err.message } });
+    }
     if (err.code && err.code.startsWith("E")) {
       return res.status(400).json({ error: { code: err.code, message: err.message } });
     }