fix(bootstrap): self-heal a systemd unit left on a retired plugin root

The residue an interrupted update leaves behind is invisible: the server
runs, every doctor row reads OK, and the unit quietly still names a cache
directory Claude Code deleted during the update. Nothing surfaces it until
a reboot, when the dashboard does not come back and nothing recorded why.

- fastPathOk now rejects a stale unit, so the next SessionStart repairs it
  instead of short-circuiting. `unitIsStale` is pure fs on purpose: that
  path runs on every session and may not spawn a process.
- Repairing the unit is not restarting the server. A current, healthy
  server keeps running and the corrected unit takes effect at the next
  start (`pendingSystemdRestart = systemdOwned && needsServer`), and the
  detached-spawn fallback is guarded by needsServer so a stale unit alone
  cannot start a second server onto a bound port.
- systemdOwned now also requires the unit to be ACTIVE. Installed-but-
  stopped owns nothing: skipping the stop there and asking systemd to start
  would bind an occupied port and crash-loop under Restart=always.
- /agenthub-doctor gains a systemd row naming the consequence ("will NOT
  come back after a reboot") and the fix. WARN, never FAIL — the dashboard
  in front of you works, and having no unit at all is a normal setup.

The fast-path test pins the regression against an otherwise fully healthy
install, live server included; without a live server it would have passed
against the unfixed code either way.

docs/PLUGINS.md

@@ -210,11 +210,33 @@ the Settings panel do — still stops it.
 it runs from your own shell, outside the unit's cgroup, so it may block and
 start in one step.
 
+**A unit pointing at a retired plugin root repairs itself on the next session.**
+This is the residue an interrupted update leaves behind, and it is invisible
+while the dashboard is up: the server runs fine, every doctor row reads OK, and
+then a reboot starts a cache directory Claude Code deleted during the update —
+so the dashboard never comes back, for a reason nothing recorded. The fast path
+therefore checks it (`unitIsStale`, pure fs — it may not spawn a process on
+every `SessionStart`) and refuses to short-circuit, and the bootstrap rewrites
+the unit.
+
+Repairing the *unit* is not restarting the *server*: when the running server is
+current and healthy, the corrected unit is simply left to take effect at the
+next start (`pendingSystemdRestart = systemdOwned && needsServer`). Restarting
+a working dashboard to fix a file that only matters at boot would be churn the
+user feels for nothing. For the same reason the detached-spawn fallback is
+guarded by `needsServer` — entering that branch for a stale unit alone would
+start a second server onto a port the live one already holds.
+
+`systemdOwned` requires the unit to be **active**, not merely present: an
+installed-but-stopped unit owns nothing, and treating it as the owner would
+skip the stop and then ask systemd to bind an occupied port — a crash loop,
+courtesy of `Restart=always`.
+
 ### Commands
 
 | Command | Does |
 |---|---|
-| `/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), UI bundle |
+| `/agenthub-doctor` | Node version, bootstrap state, runtime deps, server liveness, duplicate hooks, CLI launcher + PATH, **systemd** (`WARN` when the unit names a retired plugin root — the dashboard will not survive a reboot — or when it is installed but stopped; no unit at all is fine), **tmux** (`WARN` when absent — only the run console, `agenthub lanes shell` and session reloads need it), **skill plugins** (see below), 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) |