fix(tmux): bound calls on our own server and run them with the server's own binary

A tmux 3.8 installed under a running 3.6 server left PATH's 3.8 client
asking the 3.6 server, which spun at 100% CPU; the dashboard's
synchronous `has-session` on it had no limit and froze every page, hook
and terminal for five minutes (14:07–14:12 on the shared server).

- Every call on our own server now has a 5 s limit and the member's
  one-minute back-off (`host: tmux server not answering` meanwhile), so a
  stuck server costs the event loop one timeout per minute, not one per
  poll.
- Every call on a running server runs that server's own binary
  (`/proc/<pid>/exe`, found from its listening socket in /proc — asking
  tmux would be the very call that hangs), re-read every 10 s; PATH's
  tmux only starts a server or answers `-V`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/__tests__/tmux.test.js

@@ -344,3 +344,56 @@ describe("tmux wrapper", () => {
     assert.equal(tmux.upgradeAdvice("3.3.2", "freebsd"), null);
   });
 });
+
+describe("our own tmux server: bounded and version-matched", () => {
+  const fs = require("node:fs");
+  const os = require("node:os");
+  const path = require("node:path");
+
+  beforeEach(() => tmux.__reset());
+
+  it("backs off after a call to our server times out, instead of blocking every poll", () => {
+    let calls = 0;
+    tmux.__setExecImpl(() => {
+      calls++;
+      throw Object.assign(new Error("spawnSync tmux ETIMEDOUT"), { code: "ETIMEDOUT" });
+    });
+    assert.equal(tmux.hasSession("agenthub-lane-1"), false);
+    const after = calls;
+    assert.ok(after >= 1);
+    // Meanwhile nothing reaches tmux: the server is left alone for a minute.
+    assert.equal(tmux.hasSession("agenthub-lane-1"), false);
+    assert.equal(tmux.serverPid(), null);
+    assert.equal(calls, after);
+  });
+
+  it("finds the binary of the server listening on our socket from /proc alone", () => {
+    const root = fs.mkdtempSync(path.join(os.tmpdir(), "tmux-proc-"));
+    const sock = "/tmp/tmux-1000/default";
+    fs.mkdirSync(path.join(root, "net"));
+    fs.writeFileSync(
+      path.join(root, "net", "unix"),
+      "Num       RefCount Protocol Flags    Type St Inode Path\n" +
+        `0000: 00000003 00000000 00000000 0001 03 777 ${sock}\n` + // a client's
+        `0000: 00000002 00000000 00010000 0001 01 4242 ${sock}\n` // the listener
+    );
+    const proc = (pid, comm, fds) => {
+      fs.mkdirSync(path.join(root, pid, "fd"), { recursive: true });
+      fs.writeFileSync(path.join(root, pid, "comm"), `${comm}\n`);
+      fs.symlinkSync(process.execPath, path.join(root, pid, "exe"));
+      fds.forEach((t, i) => fs.symlinkSync(t, path.join(root, pid, "fd", String(i))));
+    };
+    proc("10", "bash", ["socket:[4242]"]); // not tmux
+    proc("11", "tmux: client", ["socket:[777]"]);
+    proc("12", "tmux: server", ["/dev/null", "socket:[4242]"]);
+    try {
+      assert.equal(
+        tmux.serverExeFor(sock, { procRoot: root, uid: null }),
+        path.join(root, "12", "exe")
+      );
+      assert.equal(tmux.serverExeFor("/tmp/other", { procRoot: root, uid: null }), null);
+    } finally {
+      fs.rmSync(root, { recursive: true, force: true });
+    }
+  });
+});