docs: document the system prerequisites, and report tmux in both doctors

The READMEs never said tmux was needed, so a fresh install only found out
when Start Run failed with nothing explaining why. Both doctors were silent
about it too.

- README/README.vi: a prerequisites table (node, npm, git, tmux, claude,
  codex, gh, systemd) and per-OS tmux install + verify steps; the feature
  list is regrouped into observe/drive/configure and now covers analytics,
  alerts, import, remote sources, the config explorer and Update all.
- INSTALL: Step 0 for the system packages, plus a troubleshooting entry
  separating "tmux missing" from "the server has a different PATH".
- SETUP/docs/README: the same troubleshooting entry and a pointer to Step 0.
- plugin-doctor: a tmux row, WARN and never FAIL - the run console and
  `lanes shell` break without it, nothing else does. Reuses tmux.js's own
  isTmuxAvailable().
- agenthub doctor: asks GET /api/run/tmux instead, because the PATH that
  decides whether a run starts is the server's, not the caller's shell's.

scripts/plugin-doctor.js

@@ -5,7 +5,8 @@
  * npm, free disk on the data dir, bootstrap state, runtime dependencies (every
  * declared package, not just that node_modules exists), server liveness,
  * duplicate hook entries (the one failure that silently doubles every token and
- * cost figure), the `agenthub` CLI launcher and its PATH, whether the slash
+ * cost figure), the `agenthub` CLI launcher and its PATH, the `tmux` binary
+ * every browser-started run depends on, whether the slash
  * commands the pipeline templates name as node drivers are actually resolvable
  * (the `superpowers` plugin, today), and the dashboard bundle — quoting the last
  * build error when it is missing.
@@ -82,6 +83,7 @@ function diagnose() {
   );
 
   add(...cliStatus());
+  add(...tmuxStatus());
   add(...skillPluginStatus());
 
   add(...dashboardUiStatus(rt));
@@ -218,6 +220,29 @@ function skillPluginStatus() {
   ];
 }
 
+/**
+ * WARN (never FAIL) when `tmux` is absent: every run the dashboard starts is a
+ * tmux session (`server/lib/tmux.js`), so without the binary the run console,
+ * `agenthub lanes shell` and the session-reload keystrokes all fail — while
+ * sessions, analytics, lanes and the CLI keep working. Same reasoning as the
+ * skill-plugins row: a partial feature loss must not fail a doctor run.
+ *
+ * This checks the doctor's own PATH. A server started as a systemd `--user`
+ * service can still see a different one — `GET /api/run/tmux` is the server's
+ * own answer, and `agenthub doctor` reports that.
+ */
+function tmuxStatus() {
+  if (require("../server/lib/tmux").isTmuxAvailable()) {
+    return ["ok", "tmux", "on PATH — the run console can start sessions"];
+  }
+  return [
+    "warn",
+    "tmux",
+    "not found — Start Run and `agenthub lanes shell` will fail (everything else is unaffected). " +
+      "Install it: sudo apt install tmux / brew install tmux (Windows: run AgentHub under WSL2)",
+  ];
+}
+
 function countLegacyHookEntries() {
   try {
     const settings = JSON.parse(fs.readFileSync(getSettingsPath(), "utf8"));