fix(updates): report a plugin build installed beside the running one, never "up to date" on a failed check

A `claude plugin update` run from a terminal (or with only the Claude desktop
app open) unpacks the new build and starts nothing: no SessionStart fires, so
the dashboard kept running the old cache dir across reboots, and with the
forge unreachable the sidebar read "Up to date" (seen on a machine stuck at
1.8.10).

- pluginStatus asks supersededBy for its own root: a recorded sibling install
  sets `installed_not_running: {root, version}` and `update_available`, with
  no network needed
- the sidebar reads "<version> installed, not running"; the update modal keys
  on it, so Update now is offered even with no release tag or sha
- plugin-update-helper hands over to the recorded install first, the newest
  sibling dir only as a fallback
- a plugin install whose release lookup failed reads "Could not check for
  updates" instead of "Up to date"
- docs/PLUGINS.md: the case and the manual hand-over command

client/src/components/UpdateNotifier.tsx

@@ -185,7 +185,13 @@ export function UpdateNotifier() {
   // also swallow v1.6.0 — and so the prompt survives the tag pointing at a
   // commit the user already has. Falls back to the upstream SHA for the
   // commit-count path (no release readable).
-  const dismissKey = status?.latest_release || status?.remote_sha || null;
+  // A build installed from a terminal while this server kept running has no
+  // tag or sha when the forge is unreachable — key it by its directory.
+  const installed = status?.installed_not_running ?? null;
+  const dismissKey =
+    status?.latest_release ||
+    status?.remote_sha ||
+    (installed ? `installed:${installed.root}` : null);
   const show = Boolean(status?.update_available && dismissKey && dismissedSha !== dismissKey);
 
   const dismiss = useCallback(() => {
@@ -320,12 +326,16 @@ export function UpdateNotifier() {
                 id="update-notifier-title"
                 className="text-sm font-semibold text-fg-primary truncate"
               >
-                {release ? t("releaseTitle", { version: release }) : t("title")}
+                {installed
+                  ? t("installedTitle", { version: installed.version ?? "?" })
+                  : release
+                    ? t("releaseTitle", { version: release })
+                    : t("title")}
               </h2>
               <p className="text-[11px] text-fg-muted mt-0.5 font-mono truncate">
                 {/* Never print a commit count on a shallow clone — the number
                     is a graft artefact, not a distance. */}
-                {release && !status.shallow_clone && status.local_version
+                {(release || installed) && !status.shallow_clone && status.local_version
                   ? t("releaseSubtitle", { current: status.local_version })
                   : t("commitsBehind", { count: behind, ref: refLabel })}
               </p>