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.

plugins/agenthub/commands/agenthub-doctor.md

@@ -1,5 +1,5 @@
 ---
-description: Diagnose a plugin-installed AgentHub — Node, npm, disk, bootstrap, server, hooks, CLI, tmux, UI build
+description: Diagnose a plugin-installed AgentHub — Node, npm, disk, bootstrap, server, hooks, CLI, systemd, 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 systemd` → when it says the unit names a **previous plugin version**,
+  the dashboard is running now but will NOT come back after a reboot. Starting
+  a new Claude Code session repairs it; print the `--force` command from the
+  output as the immediate fix. "installed but not running" just needs
+  `systemctl --user start agenthub`.
 - `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