refactor(shared): consolidate 13+ getMessageDir copies into single shared function

src/tools/background-task/message-dir.ts

@@ -1,17 +1 @@
-import { existsSync, readdirSync } from "node:fs"
-import { join } from "node:path"
-import { MESSAGE_STORAGE } from "../../features/hook-message-injector"
-
-export function getMessageDir(sessionID: string): string | null {
-  if (!existsSync(MESSAGE_STORAGE)) return null
-
-  const directPath = join(MESSAGE_STORAGE, sessionID)
-  if (existsSync(directPath)) return directPath
-
-  for (const dir of readdirSync(MESSAGE_STORAGE)) {
-    const sessionPath = join(MESSAGE_STORAGE, dir, sessionID)
-    if (existsSync(sessionPath)) return sessionPath
-  }
-
-  return null
-}
+export { getMessageDir } from "../../shared/opencode-message-dir"