Retire the chat right sidebar: chat = sessions sidebar + chat column (wave-4 item 4)

The plan now lives IN the chat (plan blocks, wave-4 item 3) and background
processes live in the activity dock (item 1), so the right sidebar's
tree panel + sub-agent fleet column is retired from the chat view:

- delete components/tree-panel.tsx (three-mode rail/list/graph panel) and
  components/sub-agents-panel.tsx; their graph interaction machinery lives
  on in plan-block.tsx and fleet observability in activity-dock.tsx.
- app/layout.tsx drops TreePanelModeProvider (the mode no longer exists).
- chat.tsx drops the xl:flex right column; the middle column is the view.
- tests: render-tree-panel.test.tsx deleted; the sub-agents panel suite
  removed from demos.test.tsx (dispatch cards' jobs-context stays — the
  central poll drives transcript dispatch cards).

app/layout.tsx

@@ -2,7 +2,6 @@ import type { Metadata } from "next";
 import "katex/dist/katex.min.css";
 import "./globals.css";
 import { AppShell, SidebarProvider } from "@/components/sidebar";
-import { TreePanelModeProvider } from "@/components/tree-panel";
 import { ActivityDock, ActivityProvider } from "@/components/activity-dock";
 
 export const metadata: Metadata = {
@@ -15,15 +14,13 @@ export default function RootLayout({ children }: { children: React.ReactNode })
 		<html lang="en" className="dark">
 			<body className="min-h-dvh antialiased">
 			<SidebarProvider>
-				<TreePanelModeProvider>
-					<ActivityProvider>
+									<ActivityProvider>
 						<AppShell>{children}</AppShell>
 						{/* The reserved bottom-right corner of every view — renders
 					    nothing while no background process runs. */}
 						<ActivityDock />
 					</ActivityProvider>
-				</TreePanelModeProvider>
-			</SidebarProvider>
+							</SidebarProvider>
 		</body>
 		</html>
 	);