Add repo-detail panel and /api/repo endpoint

New <repo-detail> component listens for repo:select and shows a repo's remotes, local branches (current + upstream), and recent commits. Backed by GET /api/repo (restricted to indexed repos) and read-only git readers LocalBranches/RecentCommits/RemoteDetails via repos.BuildDetail. <repo-list> highlights the selection; page docks the two panels left/right.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/templates/help.html

@@ -45,6 +45,14 @@
       <li><strong>Ahead / behind</strong> — commits your branch leads or trails its upstream by.</li>
     </ul>
 
+    <h2>Repository details</h2>
+    <p>
+      Click any repository in the list to open its details on the right: its
+      full path and current branch, its remotes and their URLs, every local
+      branch (with the one you're on marked and its upstream shown), and the 20
+      most recent commits.
+    </p>
+
     <h2>Background refresh</h2>
     <p>
       The dashboard refreshes on its own. By default it does <em>not</em> reach