refactor(compiler): use shared frontmatter module; fix --- boundary + concept malformed fallback

- Import openkb.frontmatter; replace local _yaml_kv_line / _yaml_list_line /
  _parse_yaml_list_value / _set_fm_line bodies with thin aliases to the shared
  module exports (call sites unchanged).
- Replace all ad-hoc text.find("---", 3) splits with frontmatter.split():
  _write_summary, _write_concept, _write_entity, _read_concept_briefs,
  _read_entity_briefs, compile_long_doc backfill, _gen_update,
  _gen_entity_update, summary-rewrite strip, _prepend_source_to_frontmatter,
  _remove_source_from_frontmatter, scan_affected_pages.
- Add module-level _resolve_description(fm) helper; use it in both reader
  functions to consolidate description/brief fallback logic.
- _read_concept_briefs and _read_entity_briefs now use frontmatter.parse()
  instead of manual yaml.safe_load.
- Replace re.sub(r"^brief:.*\n?", ...) with frontmatter.drop_line() in both
  _write_concept and _write_entity update paths.
- Add malformed-frontmatter fallback to _write_concept update path: when
  frontmatter.split returns None, rebuild valid block (type: Concept,
  sources, description) rather than writing a bare body.
- Remove now-unused direct yaml import.
- Add 3 regression tests: concept round-trip with "---" in brief, entity
  round-trip with "---" in brief, concept update on malformed frontmatter.

Author: KylinMountain