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>

internal/repos/detail.go

@@ -0,0 +1,56 @@
+package repos
+
+import (
+	"context"
+	"time"
+
+	"gitmanager/internal/git"
+)
+
+// Detail is the enriched view of a single repository shown in the detail panel.
+// It embeds the cached State and adds data fetched on demand via the git
+// boundary (all read-only — AGENT.md §1.3).
+type Detail struct {
+	State
+	Branches      []git.Branch `json:"branches"`
+	Commits       []git.Commit `json:"commits"`
+	RemoteDetails []git.Remote `json:"remoteDetails"`
+}
+
+// Get returns the cached State for a repo path, or false if it is not indexed.
+func (i *Index) Get(path string) (State, bool) {
+	i.mu.RLock()
+	defer i.mu.RUnlock()
+	s, ok := i.byKey[path]
+	return s, ok
+}
+
+// BuildDetail enriches a cached State with branches, recent commits, and remote
+// URLs. Errors on the enriching calls are non-fatal: whatever succeeds is
+// returned, and the partial failure is recorded on Detail.Error.
+func BuildDetail(ctx context.Context, g *git.CLI, base State) Detail {
+	ctx, cancel := context.WithTimeout(ctx, 20*time.Second)
+	defer cancel()
+
+	d := Detail{State: base}
+
+	if branches, err := g.LocalBranches(ctx, base.Path); err == nil {
+		d.Branches = branches
+	} else {
+		d.Error = err.Error()
+	}
+
+	if commits, err := g.RecentCommits(ctx, base.Path, 20); err == nil {
+		d.Commits = commits
+	} else if d.Error == "" {
+		d.Error = err.Error()
+	}
+
+	if remotes, err := g.RemoteDetails(ctx, base.Path); err == nil {
+		d.RemoteDetails = remotes
+	} else if d.Error == "" {
+		d.Error = err.Error()
+	}
+
+	return d
+}