fix: v2.5.6 — Windows panes without sh, UI build without node on PATH

- Windows panes: a WinGet tmux runs native programs and there is no `sh`, so
  every pane's `sh -c` wrapper died at once ("Command failed: tmux set-option
  … mouse on"). paneArgv runs the CLI itself on win32, and a Terminal gets
  tmux's own default shell (newSession drops the `--` for an empty argv).
- Plugin UI build: client deps install with --ignore-scripts and tsc/vite run
  by the running node instead of `npm run build`. On Windows esbuild's
  postinstall went through cmd.exe with no `node` on the server's PATH, both
  installs failed, and Update now kept the previous UI. A test keeps the two
  build steps equal to client/package.json's build script.
- Update all: Atomics.wait instead of a `sleep` binary Windows lacks.

server/lib/tmux.js

@@ -76,7 +76,9 @@ function envFlags(env) {
  * guessing from cwd — see resolveLaneArg in bin/agenthub.js.
  */
 function newSession({ name, cwd, argv, env }) {
-  execImpl(["new-session", "-d", "-s", name, "-c", cwd, ...envFlags(env), "--", ...argv]);
+  // No argv = tmux's own default shell (a Windows Terminal pane).
+  const cmd = argv.length ? ["--", ...argv] : [];
+  execImpl(["new-session", "-d", "-s", name, "-c", cwd, ...envFlags(env), ...cmd]);
   try {
     enableMouse(name);
   } catch (err) {
@@ -85,7 +87,9 @@ function newSession({ name, cwd, argv, env }) {
     // "Command failed: tmux set-option …" that hid the real problem.
     if (hasSession(name)) throw err;
     throw Object.assign(
-      new Error(`the session's command exited as soon as it started: ${argv[0]}`),
+      new Error(
+        `the session's command exited as soon as it started: ${argv[0] || "the default shell"}`
+      ),
       { code: "EPANEEXITED" }
     );
   }