feat(normalize): render post attachment zone in convertPost

[yjhcjykwbk-jlsec]

Summary

The rich-text attachment zone (top-level files array on a post message) was ignored by convertPost: only the locale document (title/content/content_v2) was flattened and scanned for resources, so attachments attached to a rich-text message were invisible to channel consumers (agents/CLIs).

This PR makes the attachment zone visible and downloadable:

• src/normalize/converters/post.ts
  • topLevelAttachments(): extract the top-level files: [{file_key, file_name, is_folder}] array
  • render attachment-zone entries as <file key="..." name="..."/> / <folder key="..." name="..."/> lines after the body, matching the standalone file/folder converters
  • surface attachment-zone files as ResourceDescriptor(type: 'file') so they are downloadable; folders stay tag-only (mirrors the standalone folder converter's resources: [])
• src/normalize/__tests__/converters.test.ts: two new tests — rendering + resource extraction, and empty files array tolerance

Wire shape

{
  "zh_cn": { "title": "报告", "content": [...] },
  "files": [
    { "file_key": "file_a", "file_name": "report.pdf" },
    { "file_key": "dir_1", "file_name": "assets", "is_folder": true }
  ]
}

renders as:

**报告**
正文
<file key="file_a" name="report.pdf"/>
<folder key="dir_1" name="assets"/>

with resources: [{type: 'file', fileKey: 'file_a', fileName: 'report.pdf'}].

Test

pnpm test   # 548 passed
pnpm typecheck  # clean