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/tools/delegate-task/sync-task.ts

@@ -14,6 +14,11 @@ import { buildTaskMetadataBlock } from "../../features/tool-metadata-store/task-
 import { resolveMetadataModel } from "./resolve-metadata-model"
 import { shouldRetryError } from "../../shared/model-error-classifier"
 import type { ModelFallbackState } from "../../hooks/model-fallback/hook"
+import { buildTaskPrompt } from "./prompt-builder"
+import {
+  clearDelegatedChildSessionBootstrap,
+  registerDelegatedChildSessionBootstrap,
+} from "../../shared/delegated-child-session-bootstrap"
 
 export async function executeSyncTask(
   args: DelegateTaskArgs,
@@ -36,6 +41,9 @@ export async function executeSyncTask(
     | undefined
 
   try {
+    const tddEnabled = executorCtx.sisyphusAgentConfig?.tdd
+    const delegatedPromptText = buildTaskPrompt(args.prompt, agentToUse, tddEnabled)
+
     if (typeof manager?.reserveSubagentSpawn === "function") {
       spawnReservation = await manager.reserveSubagentSpawn(parentContext.sessionID)
     }
@@ -80,11 +88,13 @@ export async function executeSyncTask(
       subagentSessions.add(newSessionID)
       syncSubagentSessions.add(newSessionID)
       setSessionAgent(newSessionID, agentToUse)
-      executorCtx.modelFallbackControllerAccessor?.setSessionFallbackChain(newSessionID, fallbackChain)
-
-      if (args.category) {
-        SessionCategoryRegistry.register(newSessionID, args.category)
-      }
+      registerDelegatedChildSessionBootstrap({
+        sessionID: newSessionID,
+        promptText: delegatedPromptText,
+        fallbackChain,
+        category: args.category,
+        modelFallbackControllerAccessor: executorCtx.modelFallbackControllerAccessor,
+      })
 
       if (onSyncSessionCreated) {
         log("[task] Invoking onSyncSessionCreated callback", { sessionID: newSessionID, parentID: parentContext.sessionID })
@@ -150,6 +160,7 @@ export async function executeSyncTask(
       sessionID,
       agentToUse,
       args,
+      promptText: delegatedPromptText,
       systemContent,
       toastManager,
       taskId,
@@ -171,6 +182,7 @@ export async function executeSyncTask(
     const cleanupRetrySession = (currentSessionID: string): void => {
       subagentSessions.delete(currentSessionID)
       syncSubagentSessions.delete(currentSessionID)
+      clearDelegatedChildSessionBootstrap(currentSessionID)
       executorCtx.modelFallbackControllerAccessor?.clearSessionFallbackChain(currentSessionID)
       SessionCategoryRegistry.remove(currentSessionID)
     }
@@ -308,6 +320,7 @@ ${buildTaskMetadataBlock({
     if (syncSessionID) {
       subagentSessions.delete(syncSessionID)
       syncSubagentSessions.delete(syncSessionID)
+      clearDelegatedChildSessionBootstrap(syncSessionID)
       executorCtx.modelFallbackControllerAccessor?.clearSessionFallbackChain(syncSessionID)
       SessionCategoryRegistry.remove(syncSessionID)
     }