feat: remove import agent feature from UI

Remove the import agent functionality from the app while preserving
the underlying resume capability in the agent code.

Changes:
- Remove ImportAgentModal and import flow from create-agent-modal.tsx
- Remove "Import Agent" menu item from agent screen
- Remove "Import" button from home footer
- Remove resumeAgent from session context and store
- Remove list_persisted_agents WS messages from server
- Remove handleListPersistedAgentsRequest handler
- Clean up unused styles and types

The resumeAgent capability remains in the agent manager for potential
future use or internal tooling.

packages/app/src/stores/session-store.ts

@@ -204,7 +204,6 @@ export interface SessionState {
       worktreeName?: string;
       requestId?: string;
     }) => Promise<void>;
-    resumeAgent: (options: { handle: any; overrides?: any; requestId?: string }) => void;
     setAgentMode: (agentId: string, modeId: string) => void;
     respondToPermission: (agentId: string, requestId: string, response: any) => void;
   } | null;