feat: add right-side activity panel for agent status

- Create ActivityPanel component showing per-agent status with
  activity dots, agent names, and contextual status labels
- Add detailPanel slot to AppShell layout (right aside, w-64)
- Move agent activity display from inline ChatPane card to the
  dedicated side panel
- Remove activity prop and inline panel from ChatPane
- Panel appears when a session is selected, hidden on WelcomePane

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/components/AppShell.tsx

@@ -3,10 +3,11 @@ import type { ReactNode } from 'react';
 interface AppShellProps {
   sidebar: ReactNode;
   content: ReactNode;
+  detailPanel?: ReactNode;
   overlay?: ReactNode;
 }
 
-export function AppShell({ sidebar, content, overlay }: AppShellProps) {
+export function AppShell({ sidebar, content, detailPanel, overlay }: AppShellProps) {
   return (
     <div className="relative flex h-screen bg-[var(--color-surface-0)] text-zinc-100">
       {/* Full-width drag region matching the title bar overlay height */}
@@ -16,6 +17,11 @@ export function AppShell({ sidebar, content, overlay }: AppShellProps) {
         {sidebar}
       </aside>
       <main className="relative min-w-0 flex-1">{content}</main>
+      {detailPanel && (
+        <aside className="flex w-64 shrink-0 flex-col border-l border-[var(--color-border)] bg-[var(--color-surface-1)]">
+          {detailPanel}
+        </aside>
+      )}
       {overlay}
     </div>
   );