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.

plugins/agenthub/commands/agenthub-doctor.md

@@ -1,5 +1,5 @@
 ---
-description: Diagnose a plugin-installed AgentHub — Node, npm, disk, bootstrap, server, hooks, CLI, UI build
+description: Diagnose a plugin-installed AgentHub — Node, npm, disk, bootstrap, server, hooks, CLI, tmux, UI build
 ---
 
 Run the diagnostic and report what it found. Do not fix anything unless the user
@@ -31,6 +31,11 @@ Print the output verbatim, then add at most three lines of guidance based on the
   means that attempt has not run or did not succeed — say `/agenthub-update` retries
   it, and print the `claude plugin install …` line from the output as the manual
   route.
+- `WARN tmux` → only the run console, `agenthub lanes shell` and the
+  session-reload keystrokes are affected; monitoring, lanes and the CLI are not.
+  Print the install command from the output. If the row says `ok` but Start Run
+  still fails, the server has a different PATH than this check — ask it directly
+  with `curl -s localhost:4820/api/run/tmux`.
 - `WARN Dashboard UI` → run `/agenthub-open` to build the bundle. When the row
   quotes a build error, report that error — it is the actual problem.