chore: delete dead AgentSidebar components (Rule 8)

QA audit found 2 unused sidebar files + 1 stale export. No imports anywhere in app/.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sonpiaz

components/dashboard-v2/agent-sidebar.tsx

@@ -83,7 +83,6 @@ export { ShortcutsHelpModal } from "./ShortcutsHelpModal";
 // ============================================
 // LAYOUT & NAVIGATION
 // ============================================
-export { AgentSidebar } from "./AgentSidebar";
 export { ExecutionTerminal } from "./ExecutionTerminal";
 export { DirectMessagesView } from "./DirectMessagesView";