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-prompt-sender.ts

@@ -57,6 +57,7 @@ export async function sendSyncPrompt(
     sessionID: string
     agentToUse: string
     args: DelegateTaskArgs
+    promptText?: string
     systemContent: string | undefined
     categoryModel: DelegatedModelConfig | undefined
     toastManager: { removeTask: (id: string) => void } | null | undefined
@@ -67,7 +68,7 @@ export async function sendSyncPrompt(
 ): Promise<string | null> {
   const allowTask = isPlanFamily(input.agentToUse)
   const tddEnabled = input.sisyphusAgentConfig?.tdd
-  const effectivePrompt = buildTaskPrompt(input.args.prompt, input.agentToUse, tddEnabled)
+  const effectivePrompt = input.promptText ?? buildTaskPrompt(input.args.prompt, input.agentToUse, tddEnabled)
   const tools = {
     task: allowTask,
     call_omo_agent: true,