The Frontmatter struct deserialization failed completely when any unknown
field had a list value (e.g. Owner: [Luca], Cadence: [Weekly]) because
serde expected a string but got an array. This caused unwrap_or_default()
to return all-None, losing type/status/archived for ~7000+ notes.
Two fixes:
1. Filter parse_frontmatter input to only known keys, preventing unknown
fields from causing deserialization failures
2. Change Owner and Cadence to StringOrList to handle both formats
85fa0b8680