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.
