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/hooks/runtime-fallback/last-user-retry-parts.ts

@@ -1,7 +1,9 @@
 import { extractSessionMessages } from "./session-messages"
+import { getDelegatedChildSessionBootstrap } from "../../shared/delegated-child-session-bootstrap"
 
 export function getLastUserRetryParts(
   messagesResponse: unknown,
+  sessionID?: string,
 ): Array<{ type: "text"; text: string }> {
   const messages = extractSessionMessages(messagesResponse)
   const lastUserMessage = messages?.filter((message) => message.info?.role === "user").pop()
@@ -9,7 +11,7 @@ export function getLastUserRetryParts(
     lastUserMessage?.parts
     ?? (lastUserMessage?.info?.parts as Array<{ type?: string; text?: string }> | undefined)
 
-  return (lastUserParts ?? [])
+  const retryParts = (lastUserParts ?? [])
     .filter(
       (part): part is { type: "text"; text: string } =>
         part.type === "text"
@@ -17,4 +19,12 @@ export function getLastUserRetryParts(
         && part.text.length > 0,
     )
     .map((part) => ({ type: "text" as const, text: part.text }))
+
+  if (retryParts.length > 0) {
+    return retryParts
+  }
+
+  return sessionID
+    ? (getDelegatedChildSessionBootstrap(sessionID)?.retryParts ?? [])
+    : []
 }