Marketplace: explain, don't offer, updates that would be refused (PR re-review)

ItemUpdate gains invalid_at_head: compute_updates records why the item
cannot be installed at head (catalog invalid, or gone) — the rule
update_marketplace_item applies — read once per marketplace from the
snapshot at head, else from the catalog parsed at head. The Installed row
shows that reason instead of a Review button, and the update counts in the
Marketplace tab and Settings count only applicable updates.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/src/components/settings/MarketplaceSettings.test.tsx

@@ -25,7 +25,19 @@ describe("MarketplaceSettings", () => {
       } as unknown as AppSettings,
     });
     listMarketplaceUpdates.mockResolvedValue([
-      { item: { marketplace_id: "m1", kind: "agent", key: "a" }, pinned: "a".repeat(40), head: "b".repeat(40) },
+      {
+        item: { marketplace_id: "m1", kind: "agent", key: "a" },
+        pinned: "a".repeat(40),
+        head: "b".repeat(40),
+        invalid_at_head: null,
+      },
+      // Not applicable (re-review round 2): not counted as available.
+      {
+        item: { marketplace_id: "m1", kind: "hook", key: "h" },
+        pinned: "a".repeat(40),
+        head: "b".repeat(40),
+        invalid_at_head: 'unknown hook event "PreFoo"',
+      },
     ]);
   });