fix(background-agent): defer parent wakes during active turns

Record fresh parent session message activity before parent-wake flushing so stale idle status cannot dispatch a background completion into a live reasoning turn.

Add regression coverage for the Discord 4.2.3/OpenCode 1.15.5 duplicate-branch repro shape where a parent reasoning delta arrives before the background all-complete wake.

Refs #4212

Refs #4019

Refs #3774

Plan: plans/background-notification-active-turn-queue.md

src/features/background-agent/parent-wake-notifier.ts

@@ -67,6 +67,7 @@ type ParentWakeNotifierOptions = {
    * inside OpenCode's `@parcel/watcher` TSFN callback path. See issue #4120.
    */
   userMessageInProgressWindowMs: number
+  parentSessionActivityInProgressWindowMs?: number
 }
 
 type ToolWaitDeferralDecision = {
@@ -94,6 +95,7 @@ export class ParentWakeNotifier {
   private pendingParentWakeTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
   private dispatchedParentWakes: Map<string, PendingParentWake> = new Map()
   private dispatchedParentWakeTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
+  private recentParentSessionActivity: Map<string, number> = new Map()
 
   constructor(
     private readonly deps: ParentWakeNotifierDeps,
@@ -116,6 +118,14 @@ export class ParentWakeNotifier {
     return this.dispatchedParentWakeTimers
   }
 
+  recordParentSessionActivity(sessionID: string): void {
+    this.recentParentSessionActivity.set(sessionID, Date.now())
+  }
+
+  clearParentSessionActivity(sessionID: string): void {
+    this.recentParentSessionActivity.delete(sessionID)
+  }
+
   queuePendingParentWake(
     sessionID: string,
     notification: string,
@@ -163,6 +173,14 @@ export class ParentWakeNotifier {
       return
     }
 
+    if (this.hasRecentParentSessionActivity(sessionID)) {
+      this.schedulePendingParentWakeFlush(sessionID)
+      log("[background-agent] Deferred parent wake because parent session activity is still fresh:", {
+        sessionID,
+      })
+      return
+    }
+
     const toolWaitDecision = await this.shouldDeferParentWakeForSessionHistory(sessionID, latestWake)
     if (toolWaitDecision.defer) {
       this.schedulePendingParentWakeFlush(sessionID)
@@ -324,12 +342,29 @@ export class ParentWakeNotifier {
     this.dispatchedParentWakeTimers.clear()
     this.pendingParentWakes.clear()
     this.dispatchedParentWakes.clear()
+    this.recentParentSessionActivity.clear()
   }
 
   private async isSessionActive(sessionID: string): Promise<boolean> {
     return isOpenCodeSessionActive(this.deps.client, sessionID)
   }
 
+  private hasRecentParentSessionActivity(sessionID: string): boolean {
+    const windowMs = this.options.parentSessionActivityInProgressWindowMs ?? 0
+    if (windowMs <= 0) {
+      return false
+    }
+    const lastActivityAt = this.recentParentSessionActivity.get(sessionID)
+    if (lastActivityAt === undefined) {
+      return false
+    }
+    if (Date.now() - lastActivityAt <= windowMs) {
+      return true
+    }
+    this.recentParentSessionActivity.delete(sessionID)
+    return false
+  }
+
   private resolveParentWakePromptContext(promptContext: ParentWakePromptContext): ParentWakePromptContext {
     const resolvedAgent = resolveRegisteredAgentName(promptContext.agent)
     return {