refactor(ai): improve error handling and stop reason types

- Add 'aborted' as a distinct stop reason separate from 'error'
- Change AssistantMessage.error to errorMessage for clarity
- Update error event to include reason field ('error' | 'aborted')
- Map provider-specific safety/refusal reasons to 'error' stop reason
- Reorganize utility functions into utils/ directory
- Rename agent.ts to agent-loop.ts for better clarity
- Fix error handling in all providers to properly distinguish abort from error

packages/ai/src/agent/index.ts

@@ -1,3 +1,3 @@
-export { prompt } from "./agent.js";
+export { agentLoop } from "./agent-loop.js";
 export * from "./tools/index.js";
 export type { AgentContext, AgentEvent, AgentTool, PromptConfig } from "./types.js";