fix(types): make AgentTrigger.config nullable to match API reality

The API can return `config: null` for non-scheduled triggers, but the
type was `Record<string, unknown>` which doesn't reflect this. Update
to `Record<string, unknown> | null` so TypeScript catches unsafe access
at compile time.

Follow-up to #415.

apps/web/shared/types/agent.ts

@@ -36,7 +36,7 @@ export interface AgentTrigger {
   id: string;
   type: AgentTriggerType;
   enabled: boolean;
-  config: Record<string, unknown>;
+  config: Record<string, unknown> | null;
 }
 
 export interface AgentTask {