refactor: deepen the Dashboard tree

CONTEXT.md

@@ -115,6 +115,8 @@ The Dashboard renders a two-level expandable tree:
 - **Stack row**: navigable; expand/collapse with `enter`/`space` to show its member PR rows. Its Stoplight is the most actionable of its members', and `f` and `o` act on the first failing member so a collapsed Stack still exposes what broke.
 - **PR row**: navigable; expand/collapse with `enter`/`space` to show that PR's Workflow runs. A PR inside a Stack renders one level deeper and carries its `position/size`. A Review glyph follows the PR number when there is a Review signal.
 
+Expansion and the cursor stay attached to what a row shows — the Repo by name, a Stack or PR by its number — so they survive PRs opening and closing, config reloads and active-first re-sorting.
+
 _Avoid_: detail view, drill-down screen
 
 ## Polling

internal/ui/autoexpand_test.go

@@ -15,7 +15,7 @@ func pr(n int, light aggregator.Stoplight) state.PRState {
 	return state.PRState{Number: n, Title: "pr", Stoplight: light}
 }
 
-func isOpen(d Dashboard, name string) bool { return d.repoExp["o/"+name] }
+func isOpen(d Dashboard, name string) bool { return d.tree.expanded[repoKey("o/"+name)] }
 
 func TestAutoExpandOpensActiveAndFailingRepos(t *testing.T) {
 	d := NewDashboard(state.New(nil))
@@ -84,8 +84,7 @@ func TestAutoExpandKeepsCursorOnSelectedRepo(t *testing.T) {
 		repoState("a", aggregator.StoplightRed, pr(1, aggregator.StoplightRed), pr(2, aggregator.StoplightRed)),
 		repoState("b", aggregator.StoplightGreen),
 	}))
-	row := d.rows[d.cursor]
-	if row.kind != kindRepo || d.snapshot.Repos[row.repoIdx].Name != "b" {
-		t.Errorf("cursor moved off repo b to %+v", row)
+	if n, _ := d.tree.selected(); n.key != repoKey("o/b") {
+		t.Errorf("cursor moved off repo b to %+v", n.key)
 	}
 }