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.

SETUP.md

@@ -324,6 +324,26 @@ The SQLite database uses WAL mode with a 5-second busy timeout. If you see lock
 
 ---
 
+### The run console won't start a run / the terminal never attaches
+
+Runs are tmux-backed: the server executes the `tmux` binary and has no fallback.
+
+```bash
+tmux -V                                               # missing → sudo apt install tmux
+curl -s http://localhost:4820/api/run/tmux            # {"available":true}
+curl -s "http://localhost:4820/api/run/binary?tool=claude"   # {"found":true,"path":"…"}
+```
+
+`{"available":false}` while your own shell finds tmux means the **server's**
+`PATH` differs from yours — typically a systemd `--user` service, which never
+reads your shell profile. Install tmux system-wide (`/usr/bin/tmux`) and
+re-run `agenthub service uninstall && agenthub service install`. The same
+applies to `claude` / `codex`: a run launches them by name.
+
+Install instructions per OS: [INSTALL.md → Step 0](./INSTALL.md#step-0--system-packages).
+
+---
+
 ### No sessions appearing after starting Claude Code
 
 **Check 1 — Is the server running?**