Drop redundant "Feed" button on the logged-in home page

The navbar already has a "Feed" entry that signed-in users see on every
page, including /. The page-header button next to "New Activity"
duplicated that path without adding anything — they pointed at the same
URL, with the navbar entry being the more discoverable of the two.

Stream D from docs/information-architecture.md.

- apps/journal/app/routes/home.tsx — remove the Feed anchor; "New
  Activity" stays as the only header CTA on the personal dashboard.
- openspec/specs/journal-landing/spec.md — retire the "Social feed
  link for signed-in users" requirement that prescribed the now-deleted
  button.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/journal/app/routes/home.tsx

@@ -167,20 +167,12 @@ export default function Home({ loaderData }: Route.ComponentProps) {
               {user.displayName ?? user.username}
             </a>
           </h1>
-          <div className="flex items-center gap-3">
-            <a
-              href="/feed"
-              className="rounded-md border border-gray-300 px-4 py-2 text-sm font-medium text-gray-700 hover:bg-gray-50"
-            >
-              {t("social.feed.title")}
-            </a>
-            <a
-              href="/activities/new"
-              className="rounded-md bg-blue-600 px-4 py-2 text-sm font-medium text-white hover:bg-blue-700"
-            >
-              {t("activities.new")}
-            </a>
-          </div>
+          <a
+            href="/activities/new"
+            className="rounded-md bg-blue-600 px-4 py-2 text-sm font-medium text-white hover:bg-blue-700"
+          >
+            {t("activities.new")}
+          </a>
         </div>
 
         {showAddPasskey && !passkeyDone && supportsPasskey === true && (