[codex] fix: escape yaml frontmatter and obsidian tags

[aa-matsunari]

Summary

• write source_nodes as escaped YAML list items
• escape Obsidian frontmatter values and slug community tags/query strings

Changed files

• graphify/ingest.py
• graphify/export.py
• tests/test_ingest.py
• tests/test_export.py

Impact

• prevents malformed YAML/frontmatter and broken Dataview queries when labels contain quotes or newlines

Tests

• /tmp/graphify-review/.venv/bin/python -m pytest /tmp/graphify-review-pr4/tests/test_ingest.py /tmp/graphify-review-pr4/tests/test_export.py