Set up Rails 8 application foundation

- Generate Rails 8.1 app with PostgreSQL and TailwindCSS
- Add docker-compose.yml (postgres:17) for local database
- Configure database.yml to connect to Dockerized Postgres via env vars
- Add Pages#home root route, application layout, and navbar partial

app/views/layouts/_navbar.html.erb

@@ -0,0 +1,19 @@
+<nav class="border-b border-gray-200 bg-white">
+  <div class="mx-auto flex max-w-5xl items-center justify-between px-4 py-3 sm:px-6 lg:px-8">
+    <%= link_to root_path, class: "flex items-center gap-2 font-semibold text-gray-900" do %>
+      <span class="inline-flex h-8 w-8 items-center justify-center rounded-md bg-gray-900 text-sm font-bold text-white">G</span>
+      <span>Git Workflow Characterization</span>
+    <% end %>
+
+    <ul class="flex items-center gap-1 text-sm font-medium text-gray-600">
+      <li>
+        <%= link_to "Home", root_path,
+              class: "rounded-md px-3 py-2 hover:bg-gray-100 hover:text-gray-900 #{'bg-gray-100 text-gray-900' if current_page?(root_path)}" %>
+      </li>
+      <li>
+        <%= link_to "Characterizations", root_path,
+              class: "rounded-md px-3 py-2 hover:bg-gray-100 hover:text-gray-900" %>
+      </li>
+    </ul>
+  </div>
+</nav>