fix(notification): suppress ready alerts during background tasks

Gate idle ready notifications on the existing background-task continuation marker so cmux does not receive premature ready alerts while delegated work is still active.

Constraint: Reuse marker state from background task lifecycle without adding new notification config

Rejected: Patch cmux directly | notification readiness belongs upstream in OMO

Confidence: high

Scope-risk: narrow

src/hooks/session-todo-status.ts

@@ -1,4 +1,5 @@
 import type { PluginInput } from "@opencode-ai/plugin"
+import { readContinuationMarker } from "../features/run-continuation-state"
 import { normalizeSDKResponse } from "../shared"
 
 interface Todo {
@@ -18,3 +19,12 @@ export async function hasIncompleteTodos(ctx: PluginInput, sessionID: string): P
     return false
   }
 }
+
+export async function hasPendingSessionWork(ctx: PluginInput, sessionID: string): Promise<boolean> {
+  const marker = readContinuationMarker(ctx.directory, sessionID)
+  if (marker?.sources["background-task"]?.state === "active") {
+    return true
+  }
+
+  return hasIncompleteTodos(ctx, sessionID)
+}