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/lib/send-rpc-request.ts

@@ -105,7 +105,6 @@ const RESPONSE_TYPE_MAP: Record<string, SessionOutboundMessage["type"]> = {
   git_repo_info_request: "git_repo_info_response",
   list_provider_models_request: "list_provider_models_response",
   list_conversations_request: "list_conversations_response",
-  list_persisted_agents_request: "list_persisted_agents_response",
   create_agent_request: "agent_state",
   refresh_agent_request: "agent_state",
   initialize_agent_request: "initialize_agent_request",