[codex] Parse public account article appmsgs

[hqhq1025]

Summary

• parse forwarded public-account article appmsgs (type=5) into readable history/export content
• expose structured article metadata under appmsg in JSON output, including url, title, description, source account, and thumbnail metadata
• make search match decompressed/formatted appmsg content so compressed public-account articles can be found by title/source/URL text

Why

Forwarded WeChat public-account articles are stored as appmsg XML. The raw XML contains the article URL and source account fields, but wx history and wx export previously collapsed these messages to only [链接] <title>, and wx search could miss compressed article appmsgs because SQL LIKE cannot inspect the decompressed body.

Validation

• cargo test (44 passed)
• cargo check
• Real local-data smoke test with an Agent Hub public-account article:
  • wx history "Agent Hub" --json --type link --since 2026-05-04 --until 2026-05-11 -n 10 now includes appmsg.url
  • wx search "候鸟300" --in "Agent Hub" --json -n 20 finds the compressed article appmsg
  • wx export "Agent Hub" --since 2026-05-04 --until 2026-05-10 -f txt includes the visible mp.weixin.qq.com URL

Notes

• cargo fmt --check currently reports formatting changes across pre-existing files on main; this PR intentionally does not reformat unrelated files.
• Cross-target checks were attempted, but this local machine lacks the required C cross toolchains/SDKs:
  • Linux target: missing x86_64-linux-gnu-gcc
  • Windows MSVC target: missing target C headers such as stdlib.h/string.h for bundled C deps