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.

server/__tests__/plugin-doctor-tmux.test.js

@@ -0,0 +1,54 @@
+/**
+ * @file Tests the `tmux` row of scripts/plugin-doctor.js — the check that tells
+ * an install its browser run console cannot work, because every run the
+ * dashboard starts is a tmux session (server/lib/tmux.js). The row must WARN
+ * and never FAIL: sessions, analytics, lanes and the CLI all work without the
+ * binary, so failing a doctor run over it would cry wolf. tmux is faked through
+ * the module's own exec seam, so the test never depends on the host having it.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+const { describe, it, after } = require("node:test");
+const assert = require("node:assert/strict");
+const fs = require("node:fs");
+const os = require("node:os");
+const path = require("node:path");
+
+const TMP_HOME = fs.mkdtempSync(path.join(os.tmpdir(), "agenthub-doctor-tmux-"));
+process.env.CLAUDE_HOME = TMP_HOME;
+
+const tmux = require("../lib/tmux");
+const { diagnose } = require("../../scripts/plugin-doctor");
+
+after(() => {
+  tmux.__reset();
+  fs.rmSync(TMP_HOME, { recursive: true, force: true });
+  delete process.env.CLAUDE_HOME;
+});
+
+/** Fake the tmux binary as present or absent, then read back the row. */
+function rowWithTmux(present) {
+  tmux.__setExecImpl(() => {
+    if (!present) throw new Error("spawnSync tmux ENOENT");
+    return "tmux 3.4\n";
+  });
+  const row = diagnose().find((r) => r.label === "tmux");
+  assert.ok(row, "the doctor must always emit a tmux row");
+  return row;
+}
+
+describe("plugin-doctor: tmux row", () => {
+  it("warns, never fails, when tmux is not on PATH", () => {
+    const row = rowWithTmux(false);
+
+    assert.equal(row.level, "warn", "a missing tmux must not fail the doctor run");
+    assert.match(row.detail, /lanes shell/, "the reader needs to know WHAT breaks");
+    assert.match(row.detail, /install/i, "and how to fix it");
+  });
+
+  it("passes when tmux is available", () => {
+    const row = rowWithTmux(true);
+
+    assert.equal(row.level, "ok", row.detail);
+  });
+});