fix(updates): give a plugin-cache install a real message, not null

`agenthub update-check` prints `message` as its headline, so every
plugin-cache install saw the literal "null" where the verdict belongs — and
that is the one install kind with no commit count to fall back on, so the
line carried nothing at all.

The plugin branch of getUpdatesStatus compares versions exactly like the
checkout branch does; it now builds the same four sentences from what it
already computed (newer release / newest / no local version to compare /
release lookup failed).

The nogit regression test asserted `message === null` as a proxy for "did
not fall through to the not-a-git-clone payload" — it now asserts that on
the message's content instead, which is what it always meant.

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

@@ -176,8 +176,15 @@ describe("getUpdatesStatus — plugin cache with no .git", () => {
 
     const s = await getUpdatesStatus(root, { latestRelease: release("v1.5.1") });
 
-    // The old behaviour: message set, everything else absent.
-    assert.equal(s.message, null, "must not fall through to the not-a-git-clone payload");
+    // The old behaviour: the not-a-git-clone message, everything else absent.
+    // Asserted on the message's CONTENT, not on it being null — a plugin
+    // install now carries its own sentence, and null was only ever a proxy for
+    // "did not fall through here".
+    assert.doesNotMatch(
+      s.message,
+      /not a git clone/i,
+      "must not fall through to the not-a-git-clone payload"
+    );
     assert.equal(s.release_check, "ok", "the release layer must actually run");
     assert.equal(s.install_kind, "plugin");
     assert.equal(s.plugin_marketplace, "sg");
@@ -208,6 +215,31 @@ describe("getUpdatesStatus — plugin cache with no .git", () => {
     assert.ok(s.plugin_marketplace && s.plugin_name && s.repo_root);
   });
 
+  it("carries a human-readable message in every outcome, never a null", async () => {
+    // `agenthub update-check` prints `message` as its headline. Null here made
+    // it print the literal "null" on every plugin-cache install — the one kind
+    // that has no commit count to fall back on.
+    const cases = [
+      [makePluginInstall("msg-newer").root, { latestRelease: release("v1.6.0") }, /v1\.6\.0/],
+      [makePluginInstall("msg-current").root, { latestRelease: release("v1.5.0") }, /newest/i],
+      [
+        makePluginInstall("msg-failed").root,
+        { latestRelease: { ok: false, reason: "unreachable" } },
+        /unreachable/,
+      ],
+      [
+        makePluginInstall("msg-noversion", { version: null }).root,
+        { latestRelease: release("v1.6.0") },
+        /no version|cannot be compared/i,
+      ],
+    ];
+    for (const [root, opts, expected] of cases) {
+      const s = await getUpdatesStatus(root, opts);
+      assert.equal(typeof s.message, "string", `message must be a string, got ${s.message}`);
+      assert.match(s.message, expected);
+    }
+  });
+
   it("does not treat an older published release as an update", async () => {
     const { root } = makePluginInstall("older");
     const s = await getUpdatesStatus(root, { latestRelease: release("v1.4.0") });

server/lib/update-check.js

@@ -402,7 +402,17 @@ async function pluginStatus(root, pluginInstall, options = {}) {
       `commands — "Update now" runs them for you and restarts the dashboard. An already-open ` +
       `Claude Code session still needs '/reload-plugins' to pick up new skills and commands.`,
     manual_command: manualParts.join(" && "),
-    message: null,
+    // A plugin install compares versions exactly like a checkout does, so it
+    // gets the same sentence. Left null, `agenthub update-check` printed the
+    // literal "null" as its headline on every plugin-cache install — the one
+    // install kind that cannot fall back to a commit count.
+    message: releaseIsNewer
+      ? `Release ${release.tag} is available (this install reports ${localVersion || "no version"}).`
+      : comparable
+        ? `Running the newest release (${release.tag}).`
+        : release.ok
+          ? `This install reports no version, so it cannot be compared with release ${release.tag}.`
+          : `Could not read the latest release (${release.reason}); version comparison unavailable.`,
 
     install_kind: "plugin",
     plugin_marketplace: pluginInstall.marketplace,