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.

scripts/systemd-service.js

@@ -63,6 +63,36 @@ function unitIsOurs(name) {
  * later Claude Code session happened to run the bootstrap. An explicit
  * `systemctl stop` (what uninstall does) still stops it for good.
  */
+/**
+ * Does our unit still point at `root`? A plugin update moves the install to a
+ * new cache directory, and a unit left naming the old one is a failure that
+ * shows NOTHING until the next reboot — the dashboard simply does not come
+ * back, from a path Claude Code garbage-collected weeks earlier.
+ *
+ * Pure fs, no `systemctl`: this is called from the bootstrap's fast path, which
+ * runs on every single SessionStart and may not spawn a process.
+ *
+ * A foreign unit is never "stale" — it is not ours to judge or rewrite.
+ */
+function unitIsStale(name, root) {
+  let body;
+  try {
+    body = fs.readFileSync(unitPath(name), "utf8");
+  } catch {
+    return false; // no unit at all: nothing to be stale
+  }
+  if (!body.includes(MARKER)) return false;
+  return !body.includes(`\nWorkingDirectory=${root}\n`);
+}
+
+/** Is the unit currently running? Costs a `systemctl` call — never call it
+ *  from the fast path. */
+function unitIsActive(name) {
+  if (!systemctlAvailable()) return false;
+  const res = execImpl(["is-active", `${name}.service`], { encoding: "utf8", stdio: "pipe" });
+  return String(res.stdout || "").trim() === "active";
+}
+
 function unitContent({ description, workingDir, execStart, env }) {
   const envLines = Object.entries(env)
     .map(([k, v]) => `Environment=${k}=${v}`)
@@ -178,6 +208,8 @@ function enableLinger() {
 module.exports = {
   systemctlAvailable,
   unitIsOurs,
+  unitIsStale,
+  unitIsActive,
   writeUserService,
   restartUserService,
   installUserService,