feat(04-01): create notification service with batching, urgency, and GC protection

- Add createNotificationService with 10s batch window, urgency mapping, click-to-navigate
- Extend DesktopEvent union with NotificationClickEvent type
- Add 10 unit tests covering batching, urgency, NEVER_BATCH, GC protection, Linux urgency
- Platform-specific: silent flag, Windows timeoutType, Linux urgency level

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/shared/types.ts

@@ -376,6 +376,12 @@ export interface NewMessageEvent {
   message: AgentMessageRecord;
 }
 
+export interface NotificationClickEvent {
+  type: "notification-click";
+  section: SectionId;
+  entityId?: string;
+}
+
 export type DesktopEvent =
   | LogChunkEvent
   | DomainChangedEvent
@@ -384,7 +390,8 @@ export type DesktopEvent =
   | HeartbeatStatusEvent
   | AgentMessageEvent
   | AutomationFiredEvent
-  | NewMessageEvent;
+  | NewMessageEvent
+  | NotificationClickEvent;
 
 export type InboxItemType = "pending_approval" | "failed_run" | "budget_warning" | "blocked_task" | "review_needed" | "agent_message";