fix: parse multi-line YAML block scalars in frontmatter

_parse_frontmatter used manual line splitting which only handled
simple key: value pairs. Multi-line block scalars (|) returned
only '|' as the value, dropping all subsequent indented lines.

Replace with yaml.safe_load() to correctly parse all YAML syntax.
Add pyyaml>=6.0 to requirements.txt.

requirements.txt

@@ -1,2 +1,3 @@
 anthropic>=0.25.0
 python-dotenv>=1.0.0
+pyyaml>=6.0