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/Sidebar.tsx

@@ -368,15 +368,25 @@ export function Sidebar({ wsConnected, collapsed, onToggle }: SidebarProps) {
     ? t("nav:checkingForUpdates")
     : checkError
       ? t("nav:checkFailed")
-      : updateAvailable
-        ? updateStatus?.latest_release
-          ? t("nav:releaseAvailable", { version: updateStatus.latest_release })
-          : t("nav:updateAvailable")
-        : updateStatus?.dev_build
-          ? t("nav:devBuild", { count: updateStatus.commits_ahead_of_release ?? 0 })
-          : updateStatus
-            ? t("nav:upToDate")
-            : t("nav:checkForUpdates");
+      : updateStatus?.installed_not_running
+        ? t("nav:installedNotRunning", {
+            version: updateStatus.installed_not_running.version ?? "?",
+          })
+        : updateAvailable
+          ? updateStatus?.latest_release
+            ? t("nav:releaseAvailable", { version: updateStatus.latest_release })
+            : t("nav:updateAvailable")
+          : updateStatus?.dev_build
+            ? t("nav:devBuild", { count: updateStatus.commits_ahead_of_release ?? 0 })
+            : // A plugin install has no commit count to fall back on: a release
+              // that could not be read is "unknown", never "up to date".
+              updateStatus?.install_kind === "plugin" &&
+                updateStatus.release_check &&
+                updateStatus.release_check !== "ok"
+              ? t("nav:releaseUnchecked")
+              : updateStatus
+                ? t("nav:upToDate")
+                : t("nav:checkForUpdates");
   const currentLanguage = normalizeLanguage(i18n.resolvedLanguage ?? i18n.language);
   const currentIndex = SUPPORTED_LANGUAGES.indexOf(currentLanguage);
   const nextLanguage = SUPPORTED_LANGUAGES[(currentIndex + 1) % SUPPORTED_LANGUAGES.length];

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>

client/src/components/__tests__/Sidebar.test.tsx

@@ -5,10 +5,11 @@
  */
 
 import { describe, it, expect } from "vitest";
-import { render, screen, waitFor } from "@testing-library/react";
+import { act, render, screen, waitFor } from "@testing-library/react";
 import userEvent from "@testing-library/user-event";
 import { MemoryRouter } from "react-router-dom";
 import { Sidebar } from "../Sidebar";
+import { eventBus } from "../../lib/eventBus";
 
 function renderSidebar(wsConnected: boolean, collapsed = false) {
   return render(
@@ -103,4 +104,44 @@ describe("Sidebar", () => {
       expect(screen.getByText("VI")).toBeInTheDocument();
     });
   });
+
+  describe("update status line", () => {
+    const plugin = {
+      git_repo: false,
+      install_kind: "plugin",
+      situation: "plugin_managed",
+      local_version: "1.8.10",
+    };
+    const publish = (data: object) =>
+      act(() => eventBus.publish({ type: "update_status", data } as never));
+
+    it("does not claim up to date when a plugin install could not read the release", () => {
+      renderSidebar(true);
+      publish({ ...plugin, update_available: false, release_check: "unreachable" });
+      expect(screen.queryByText("Up to date")).toBeNull();
+      expect(screen.getByText("Could not check for updates")).toBeInTheDocument();
+    });
+
+    it("names a build installed beside the running one", () => {
+      renderSidebar(true);
+      publish({
+        ...plugin,
+        update_available: true,
+        release_check: "unreachable",
+        installed_not_running: { root: "/x", version: "1.8.12" },
+      });
+      expect(screen.getByText("1.8.12 installed, not running")).toBeInTheDocument();
+    });
+
+    it("still says up to date when the release was read and matches", () => {
+      renderSidebar(true);
+      publish({
+        ...plugin,
+        update_available: false,
+        release_check: "ok",
+        latest_release: "v1.8.10",
+      });
+      expect(screen.getByText("Up to date")).toBeInTheDocument();
+    });
+  });
 });

client/src/components/__tests__/UpdateNotifier.test.tsx

@@ -137,6 +137,33 @@ describe("UpdateNotifier — Update now", () => {
     expect(screen.queryByText(i18n.t("updates:notGit"))).not.toBeInTheDocument();
   });
 
+  it("offers Update now for a build installed from a terminal, even with no release read", async () => {
+    // `claude plugin update` started nothing and the forge was unreachable:
+    // no release tag and no sha to key the modal on, yet a newer build sits
+    // on disk. That used to read as "up to date" with no way to switch.
+    statusMock.mockResolvedValue({
+      ...BASE_STATUS,
+      git_repo: false,
+      situation: "plugin_managed",
+      install_kind: "plugin",
+      plugin_marketplace: "sg",
+      plugin_name: "agenthub",
+      remote_sha: null,
+      commits_behind: 0,
+      local_version: "1.8.10",
+      release_check: "unreachable",
+      latest_release: null,
+      installed_not_running: { root: "/cache/sg/agenthub/new", version: "1.8.12" },
+    });
+    render(<UpdateNotifier />);
+
+    await screen.findByText(i18n.t("updates:installedTitle", { version: "1.8.12" }));
+    expect(
+      screen.getByText(i18n.t("updates:releaseSubtitle", { current: "1.8.10" }))
+    ).toBeInTheDocument();
+    expect(screen.getByText(i18n.t("updates:updateNow"))).toBeInTheDocument();
+  });
+
   it("surfaces a previous plugin chain's failure so a dead update isn't silent", async () => {
     statusMock.mockResolvedValue({
       ...BASE_STATUS,