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>

CHANGELOG.md

@@ -14,6 +14,19 @@ Append-only running history of all changes (AGENT.md §9.1). Newest last.
 - **Why:** Stand up the architecture defined in AGENT.md so feature work can begin.
 - **Affects:** whole repo (foundation); `components/repo-list`.
 
+## 2026-09-19 — Repo detail panel
+- **What:** Added the `<repo-detail>` component (right dock) that listens for
+  `repo:select` and shows a repo's remotes, local branches (current + upstream),
+  and 20 most recent commits. Backed by a new `GET /api/repo?path=` endpoint
+  (restricted to indexed repos) and new read-only git readers
+  (`LocalBranches`, `RecentCommits`, `RemoteDetails`) plus `repos.BuildDetail`
+  and `Index.Get`. `<repo-list>` now highlights the selected repo; `index.html`
+  lays the two panels out left/right; help page documents the detail view.
+- **Why:** Make the dashboard drill into a single repository (the detail half of
+  the list+detail default in AGENT.md §4).
+- **Affects:** `components/repo-detail`, `components/repo-list`,
+  `internal/git`, `internal/repos`, `cmd/server`, `web/templates`.
+
 ### Notes to confirm (from AGENT.md §11)
 - **Go module path** is the placeholder `gitmanager`; change it if this gets a
   canonical import path (e.g. a GitHub URL).