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/CLI.md

@@ -272,7 +272,7 @@ Because a lane's services are fully detached, restarting the dashboard (or `agen
 
 | Command | Description |
 | ------- | ----------- |
-| `agenthub doctor` | Diagnosis: API reachability, hook installation status + path, database path/size/row counts, server uptime and Node version, WS connections |
+| `agenthub doctor` | Diagnosis: API reachability, hook installation status + path, database path/size/row counts, server uptime and Node version, WS connections, whether `tmux` is on **the server's** PATH (asked via `GET /api/run/tmux` — a systemd `--user` service does not read your shell profile), remote-source status |
 | `agenthub info` | The raw `/api/settings/info` JSON (pipe it to `jq`) |
 | `agenthub export [file.json]` | Full JSON data export (sessions, agents, events, tokens, workflows, dashboard runs, alert rules, pricing) — defaults to a dated filename. Re-importable via `agenthub import-data` |
 | `agenthub cleanup --hours N --days M` | Abandon active sessions idle for `N` hours and/or purge completed sessions older than `M` days |

docs/PLUGINS.md

@@ -176,7 +176,7 @@ reboot, same as always.
 
 | Command | Does |
 |---|---|
-| `/agenthub-doctor` | Node version, bootstrap state, runtime deps, server liveness, duplicate hooks, CLI launcher + PATH, **skill plugins** (see below), MCP build freshness, UI bundle |
+| `/agenthub-doctor` | Node version, bootstrap state, runtime deps, server liveness, duplicate hooks, CLI launcher + PATH, **tmux** (`WARN` when absent — only the run console, `agenthub lanes shell` and session reloads need it), **skill plugins** (see below), MCP build freshness, UI bundle |
 | `/agenthub-update` | Pull the latest `agenthub` release via `claude plugin marketplace update` + `claude plugin update agenthub@<marketplace> -y`, then reinstall dependencies and restart the server (`plugin-bootstrap.js --force`). The plugin pull needs a manual `/reload-plugins` afterward to take effect in the current session — not scriptable from a command. |
 | `/agenthub-open` | Rebuild the UI bundle and print the dashboard URL. Re-runs the bootstrap first when the server is down or still executing a plugin root that a mid-session `/plugin` update retired, so a fresh bundle never talks to an older server's routes |
 | `/ship-full <requirement>` | Put the lane on the 16-node `ship-feature` template, then run `/agenthub:ship-feature-lane` **in this session**. The in-session twin of `agenthub ship-full` (which spawns its own `claude` from a terminal) |

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`
 
 ---