fix(hooks): respect previous message's agent mode in message sending hooks

Message hooks like todo-continuation-enforcer and background-notification
now preserve the agent mode from the previous message when sending follow-up
prompts. This ensures that continuation messages and task completion
notifications use the same agent that was active in the conversation.

- Export findNearestMessageWithFields and MESSAGE_STORAGE from hook-message-injector
- Add getMessageDir helper to locate session message directories
- Pass agent field to session.prompt in todo-continuation-enforcer
- Pass agent field to session.prompt in BackgroundManager.notifyParentSession

Closes #59

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src/features/hook-message-injector/index.ts

@@ -1,2 +1,4 @@
-export { injectHookMessage } from "./injector"
+export { injectHookMessage, findNearestMessageWithFields } from "./injector"
+export type { StoredMessage } from "./injector"
 export type { MessageMeta, OriginalMessageContext, TextPart } from "./types"
+export { MESSAGE_STORAGE } from "./constants"

src/features/hook-message-injector/injector.ts

@@ -3,13 +3,13 @@ import { join } from "node:path"
 import { MESSAGE_STORAGE, PART_STORAGE } from "./constants"
 import type { MessageMeta, OriginalMessageContext, TextPart } from "./types"
 
-interface StoredMessage {
+export interface StoredMessage {
   agent?: string
   model?: { providerID?: string; modelID?: string }
   tools?: Record<string, boolean>
 }
 
-function findNearestMessageWithFields(messageDir: string): StoredMessage | null {
+export function findNearestMessageWithFields(messageDir: string): StoredMessage | null {
   try {
     const files = readdirSync(messageDir)
       .filter((f) => f.endsWith(".json"))