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.

docs/README.md

@@ -70,10 +70,12 @@ graph LR
 
 **Quick Start:**
 
-1. Read [SETUP.md](../SETUP.md)
-2. Run `npm run setup`
-3. Run `npm run dev`
-4. Open browser to `http://localhost:5173`
+1. Install the system packages — `node` (20+), `git`, and `tmux` (the run
+   console has no fallback without it): [INSTALL.md → Step 0](../INSTALL.md#step-0--system-packages)
+2. Read [SETUP.md](../SETUP.md)
+3. Run `npm run setup`
+4. Run `npm run dev`
+5. Open browser to `http://localhost:5173`
 
 ---