Current time in YYYY-MM-DDTHH:MM:SSZ form, derived directly from
SystemTime to avoid a chrono dependency where one is not otherwise
needed. Used for the pulled_at front-matter stamp.
Parse a --since-style value. Accepts either a relative duration
(7d, 24h, 30m, 1w, 90s) or an ISO-8601 absolute timestamp
(2026-04-01, 2026-04-01T12:00:00Z). Returns the resulting cutoff
instant (now - duration, or the ISO value itself).
Quote a YAML scalar if it contains characters that would confuse the
parser. Keeps simple values unquoted. Shared by every command that
writes YAML front matter (topic pull --full, category pull).