[Feature] Add task:get_cta_urls() API for proper CTA domain extraction

[Feature] Add task:get_cta_urls() API for proper CTA domain extraction

- C code (message.c): collect top CTA URLs per HTML part by button weight,
  store in task mempool variable "html_cta_urls"
- Lua API (lua_task.c): add task:get_cta_urls([max_urls]) method
- llm_search_context: use new API instead of reimplementing CTA logic in Lua
- Benefits: single source of truth for CTA logic, uses C knowledge of HTML
  structure and button weights, cleaner Lua code

This provides proper architecture where C code handles HTML structure analysis
and Lua adds domain filtering (blacklists, infrastructure domains, etc.)