Remove owner, cadence, created_at, and created_time as hardcoded fields from
the Frontmatter struct — they don't drive app logic and belong in generic
properties. Owner and cadence values now flow through to the properties map
(including single-element array unwrapping). Creation date is sourced from
filesystem metadata (birthtime on macOS) instead of frontmatter.
Also fixes title extraction: add H1 heading extraction to extract_title()
(priority: frontmatter title → H1 → filename slug). Previously titles fell
back directly to filename when no frontmatter title was set.
StringOrList is retained for scalar Frontmatter fields as a defensive measure:
YAML values can arrive as single-element arrays (e.g. Status: [Active]) which
would cause entire deserialization to fail without it.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
e6b3278fea