fix(delegate-task): harden child-session fallback bootstrap and cleanup

Capture delegated child-session retry context before the first prompt so fallback recovery still works when session history is empty. Align background and sync launch paths around the same bootstrap contract, clear session-scoped fallback state on every terminal path, and lock the behavior with regression coverage for first-prompt retries, exhaustion, isolation, and cleanup.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/manager.ts

@@ -87,6 +87,10 @@ import {
   resolveSubagentSpawnContext,
   type SubagentSpawnContext,
 } from "./subagent-spawn-limits"
+import {
+  clearDelegatedChildSessionBootstrap,
+  registerDelegatedChildSessionBootstrap,
+} from "../../shared/delegated-child-session-bootstrap"
 
 type OpencodeClient = PluginInput["client"]
 
@@ -249,6 +253,12 @@ export class BackgroundManager {
     }
   }
 
+  private cleanupDelegatedSessionContext(sessionID: string): void {
+    clearDelegatedChildSessionBootstrap(sessionID)
+    this.modelFallbackControllerAccessor?.clearSessionFallbackChain(sessionID)
+    SessionCategoryRegistry.remove(sessionID)
+  }
+
   async assertCanSpawn(parentSessionID: string): Promise<SubagentSpawnContext> {
     const spawnContext = await resolveSubagentSpawnContext(this.client, parentSessionID, this.directory)
     const maxDepth = getMaxSubagentDepth(this.config)
@@ -618,6 +628,14 @@ export class BackgroundManager {
       return
     }
 
+    registerDelegatedChildSessionBootstrap({
+      sessionID,
+      promptText: input.prompt,
+      fallbackChain: input.fallbackChain,
+      category: input.category,
+      modelFallbackControllerAccessor: this.modelFallbackControllerAccessor,
+    })
+
     task.progress = {
       toolCalls: 0,
       lastUpdate: new Date(),
@@ -778,6 +796,7 @@ The fallback retry session is now created and can be inspected directly.
         // Abort the session to prevent infinite polling hang
         // Awaited to prevent dangling promise during subagent teardown (Bun/WebKit SIGABRT)
         await this.abortSessionWithLogging(sessionID, "launch error cleanup")
+        this.cleanupDelegatedSessionContext(sessionID)
 
         this.markForNotification(existingTask)
         this.enqueueNotificationForParent(existingTask.parentSessionId, () => this.notifyParentSession(existingTask)).catch(err => {
@@ -1417,7 +1436,7 @@ The fallback retry session is now created and can be inspected directly.
       }
 
       this.rootDescendantCounts.delete(sessionID)
-      SessionCategoryRegistry.remove(sessionID)
+      this.cleanupDelegatedSessionContext(sessionID)
     }
 
     if (event.type === "session.status") {
@@ -1521,7 +1540,7 @@ The fallback retry session is now created and can be inspected directly.
     }
     this.scheduleTaskRemoval(task.id)
     if (task.sessionId) {
-      SessionCategoryRegistry.remove(task.sessionId)
+      this.cleanupDelegatedSessionContext(task.sessionId)
     }
 
     this.markForNotification(task)
@@ -1571,6 +1590,7 @@ The task was re-queued on a fallback model after a retryable failure.
         this.clearSessionOutputObserved(previousSessionID)
         this.clearSessionTodoObservation(previousSessionID)
         subagentSessions.delete(previousSessionID)
+        this.cleanupDelegatedSessionContext(previousSessionID)
       }
       return retried
     })
@@ -1743,7 +1763,7 @@ The task was re-queued on a fallback model after a retryable failure.
       this.clearTaskHistoryWhenParentTasksGone(task.parentSessionId)
       if (task.sessionId) {
         subagentSessions.delete(task.sessionId)
-        SessionCategoryRegistry.remove(task.sessionId)
+        this.cleanupDelegatedSessionContext(task.sessionId)
       }
       log("[background-agent] Removed completed task from memory:", taskId)
     }, TASK_CLEANUP_DELAY_MS)
@@ -1818,7 +1838,7 @@ The task was re-queued on a fallback model after a retryable failure.
       // Awaited to prevent dangling promise during subagent teardown (Bun/WebKit SIGABRT)
       await this.abortSessionWithLogging(task.sessionId, `task cancellation (${source})`)
 
-      SessionCategoryRegistry.remove(task.sessionId)
+      this.cleanupDelegatedSessionContext(task.sessionId)
     }
 
     removeTaskToastTracking(task.id)
@@ -1938,7 +1958,7 @@ The task was re-queued on a fallback model after a retryable failure.
       // Awaited to prevent dangling promise during subagent teardown (Bun/WebKit SIGABRT)
       await this.abortSessionWithLogging(task.sessionId, `task completion (${source})`)
 
-      SessionCategoryRegistry.remove(task.sessionId)
+      this.cleanupDelegatedSessionContext(task.sessionId)
     }
 
     try {
@@ -2236,7 +2256,7 @@ The task was re-queued on a fallback model after a retryable failure.
     removeTaskToastTracking(task.id)
     this.scheduleTaskRemoval(task.id)
     if (task.sessionId) {
-      SessionCategoryRegistry.remove(task.sessionId)
+      this.cleanupDelegatedSessionContext(task.sessionId)
     }
 
     this.markForNotification(task)
@@ -2414,7 +2434,7 @@ The task was re-queued on a fallback model after a retryable failure.
 
     for (const sessionID of trackedSessionIDs) {
       subagentSessions.delete(sessionID)
-      SessionCategoryRegistry.remove(sessionID)
+      this.cleanupDelegatedSessionContext(sessionID)
     }
 
     this.concurrencyManager.clear()