refactor(agent): rename md_to_html to readme_processor, fix section extraction, widget parsing, and list_base_models

- Rename md_to_html.py → readme_processor.py (file no longer just HTML conversion)
- _extract_section: include YAML frontmatter, use heading-level-aware forward
  walk (sub-headings under # are included), increase walk limit past 30 lines
- _is_heading: exclude </hN> closing tags from boundary detection
- _heading_level: new helper for heading-level-aware section matching
- css: yield 0 for heading like closing tags, was unexpectedly caught by _is_heading
- extract_gallery_images: fix YAML block scalar (text: >-) prompt extraction;
  use endswith instead of == to detect the block marker
- _strip_widget_section: add to clean_readme_for_llm (widget text is handled
  by post-processor, not needed in LLM prompt)
- _strip_standalone_images: keep markdown image URLs intact for LLM preview
  extraction (was stripping to alt text only)
- list_base_models: switch from scanner-cache aggregation to
  CivitaiBaseModelService.get_base_models() - always returns full list
- Ollama: add num_ctx=32768 to payload options so thinking models have room
  to both reason and produce output
- Add tests/agent_cli/test_readme_processor.py: 59 tests covering extraction,
  cleaning, section matching, heading detection
- Update existing tests for behavioral changes

tests/enrich_hf_validation/report_generator.py

@@ -128,7 +128,7 @@ def generate_optimisation_suggestions(
     if prev and prev.get("empty_rate_pct", 0) > 50:
         suggestions.append(
             "- **预览图下载成功率低 ({:.0f}%)**: 很多 HF 模型卡没有 embed 图片（仅使用 YAML widget "
-            "或 external link）。当前 `md_to_html.py` 的 `extract_gallery_images` 和 "
+            "或 external link）。当前 `readme_processor.py` 的 `extract_gallery_images` 和 "
             "`extract_gallery_table_images` 已覆盖了多数场景。若预览图不重要，可降低此字段权重。".format(
                 prev.get("empty_rate_pct", 0)
             )