Fleet panel gains a collapsed rail view in lockstep with the plan

The right sidebar's mode (rail | list | graph) is now shared state: a
new useTreePanelMode() hook lets every sidebar section collapse and
expand together. With sub-agents running in rail mode the fleet shows a
quiet strip — bot icon (click expands the sidebar) + vertical 'N running'
count — mirroring the plan rail's node-count strip below it. The sidebar
now collapses all the way regardless of running jobs.

End-to-end verified through the real flow (dispatch part in the
transcript + running job record, the 1s jobs poll rendering the panel):
rail column exactly w-10 with both rails stacked and aligned (40px), the
fleet rail's expand click transitions to list mode and persists, fleet +
plan panels equal widths in list (256/256) and graph (384/384) modes,
9-node graph intact, console clean.

components/tree-panel.tsx

@@ -179,6 +179,12 @@ const TreePanelModeContext = createContext<{ mode: PanelMode; go: (next: PanelMo
 	go: () => {},
 });
 
+/** The right sidebar's shared mode — every section of the sidebar (plan,
+ * sub-agents) collapses and expands in lockstep through this hook. */
+export function useTreePanelMode() {
+	return useContext(TreePanelModeContext);
+}
+
 /** Root-layout persistence of the panel mode (rail | list | graph). */
 export function TreePanelModeProvider({ children }: { children: React.ReactNode }) {
 	// SSR default: "list" — server and client first-render agree (no
@@ -220,7 +226,7 @@ export function TreePanel({
 	messages: readonly TreePanelMessage[];
 }) {
 	const tree = useMemo(() => latestTree(messages), [messages]);
-	const { mode, go } = useContext(TreePanelModeContext);
+	const { mode, go } = useTreePanelMode();
 
 	// Empty state: no tree yet → the panel does not render at all. It appears
 	// the moment plan_init's output lands and updates live from there.