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.

bin/agenthub.js

@@ -1279,6 +1279,23 @@ async function cmdDoctor() {
   );
   console.log(`${c.green("✔")}  WS connections  ${srv.ws_connections ?? 0}`);
 
+  // Asked of the SERVER, not of this shell: a run is a tmux session the server
+  // spawns, so the only PATH that decides whether Start Run works is its own —
+  // a systemd --user service never reads your shell profile. Missing tmux is a
+  // warning, not a failure: it costs the run console and `lanes shell`, nothing
+  // else.
+  try {
+    const { available } = await get("/api/run/tmux");
+    console.log(
+      available
+        ? `${c.green("✔")}  tmux  available to the server`
+        : `${c.yellow("!")}  tmux  not on the server's PATH — Start Run and \`agenthub lanes shell\` will fail. ` +
+            `Install it (sudo apt install tmux / brew install tmux), then restart the server`
+    );
+  } catch {
+    console.log(`${c.dim("·")}  tmux  not reported by this server`);
+  }
+
   try {
     const { sources = [] } = await get("/api/remote-sources");
     if (sources.length === 0) {
@@ -2995,7 +3012,7 @@ const COMMAND_GROUPS = [
   [
     "Administration",
     [
-      ["doctor", "", "Connectivity, hooks, database, and remote sources diagnosis"],
+      ["doctor", "", "Connectivity, hooks, database, tmux, and remote sources diagnosis"],
       ["info", "", "Raw system info JSON"],
       ["export", "[file.json]", "Export all data as JSON"],
       ["cleanup", "--hours N --days M", "Abandon stale / purge old sessions"],