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

server/__tests__/update-check-plugin-nogit.test.js

@@ -384,3 +384,39 @@ describe("fetchReleaseFromUrl", () => {
     }
   });
 });
+
+describe("a newer build installed beside the running one", () => {
+  /** Record `installed` as Claude Code's install of agenthub@sg. */
+  function recordInstall(home, installed) {
+    fs.writeFileSync(
+      path.join(home, "plugins", "installed_plugins.json"),
+      JSON.stringify({ plugins: { "agenthub@sg": [{ scope: "user", installPath: installed }] } })
+    );
+  }
+
+  it("reports an update even when the release check fails", async () => {
+    // `claude plugin update` from a terminal unpacks the new build and starts
+    // nothing: the server keeps running the old directory, and with the forge
+    // unreachable it used to answer "up to date" forever.
+    const { root, home } = makePluginInstall("installed-newer", { version: "1.8.10" });
+    const newer = path.join(path.dirname(root), "aaaaaaaaaaaa");
+    fs.mkdirSync(newer, { recursive: true });
+    fs.writeFileSync(path.join(newer, "package.json"), JSON.stringify({ version: "1.8.12" }));
+    recordInstall(home, newer);
+
+    const s = await getUpdatesStatus(root, {
+      latestRelease: { ok: false, reason: "unreachable" },
+    });
+    assert.equal(s.update_available, true);
+    assert.deepEqual(s.installed_not_running, { root: newer, version: "1.8.12" });
+    assert.match(s.message, /1\.8\.12 is installed but not running/);
+  });
+
+  it("says nothing extra when the running build is the installed one", async () => {
+    const { root, home } = makePluginInstall("installed-same", { version: "1.8.12" });
+    recordInstall(home, root);
+    const s = await getUpdatesStatus(root, { latestRelease: release("v1.8.12") });
+    assert.equal(s.update_available, false);
+    assert.equal(s.installed_not_running, null);
+  });
+});