feat: replace custom frontmatter parsing with linked-markdown-py

[EthanThatOneKid]

Summary

Replaces wiki's in-house frontmatter parsing (\src/wiki/parser.py, \src/wiki/document.py) with the canonical reference library \linked-markdown-py\ (wazootech/linked-markdown-py).

Closes #204.

Changes

• parser.py: \parse_frontmatter()\ now delegates to \linked_markdown_py.extract()\ — supports all LMD spec delimiter variants (---, +++, = yaml =, ---yaml, ---json, ---toml) and formats (YAML, JSON, TOML). Error handling remains backward-compatible (returns \None\ on no frontmatter or parse failure).
• document.py: \split_frontmatter_text()\ uses LMD for attrs parsing instead of the old \parse_frontmatter()\ function. Text-level split logic (raw ---\ delimiter) unchanged.
• graph.py: _process_document_file()\ body extraction uses \extract().body\ instead of raw ---\ split, ensuring correct body extraction for all delimiter styles.
• pyproject.toml: Added \linked-markdown-py\ dependency (git source until published to PyPI). Removed \pyyaml\ as a direct dependency (now transitive through LMD).

Testing

All 518 existing tests pass with zero regressions.

Notes

• \\pyyaml\\ remains a direct dependency for non-markdown data files (.yaml, .yml) — these are not frontmatter and are not covered by LMD.
• Once \linked-markdown-py\ is published to PyPI, the [tool.uv.sources]\ override can be removed.
• The TypeScript NPM wrapper (
  pm/src/) is unchanged — this migration is Python-side only.