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).

components/chat.tsx

@@ -25,8 +25,6 @@ import { PlanBlock, PlanPopup } from "@/components/plan-block";
 import type { PlanBlockData } from "@/src/core";
 import { QuestionDock } from "@/components/question-dock";
 import { SidebarToggle } from "@/components/sidebar";
-import { SubAgentsPanel } from "@/components/sub-agents-panel";
-import { TreePanel } from "@/components/tree-panel";
 import type { ArtifactPartProps, JobRecord, Phase3, QuestionAnswerData, TeacherUIMessage, TurnUsage } from "@/src/core";
 import { PhaseChip, PhaseDivider, planNodeCount } from "@/components/phase-divider";
 import { findPendingToolCall, phaseAt3, phaseTimeline } from "@/src/core";
@@ -471,13 +469,6 @@ export function Chat({
 				</Conversation>
 				</div>
 			</div>
-			{/* Right sidebar, a full-height sibling of the middle column: the
-			    live sub-agent fleet (when running) above the plan tree — the
-			    fleet panel renders nothing while idle, so the column is all plan. */}
-			<div className="hidden shrink-0 flex-col xl:flex">
-				<SubAgentsPanel jobs={jobs} />
-				<TreePanel messages={messages} />
-			</div>
 		</div>
 		{planOpen && <PlanPopup messages={messages} onClose={() => setPlanOpen(false)} />}
 		</JobsContext.Provider>