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/test/handoff.test.ts

@@ -238,7 +238,7 @@ const providerContexts = {
 				cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
 			},
 			stopReason: "error",
-			error: "Request was aborted",
+			errorMessage: "Request was aborted",
 		} satisfies AssistantMessage,
 		toolResult: null,
 		facts: {
@@ -293,7 +293,7 @@ async function testProviderHandoff<TApi extends Api>(
 
 		// Check for error
 		if (response.stopReason === "error") {
-			console.log(`[${sourceLabel} → ${targetModel.provider}] Failed with error: ${response.error}`);
+			console.log(`[${sourceLabel} → ${targetModel.provider}] Failed with error: ${response.errorMessage}`);
 			return false;
 		}