parseDuration fell back to an unanchored search for number-and-unit
pieces when time.ParseDuration failed, and skipped everything in
between. 1.0y became 0, so `snapshot create --prune --keep-newer-than
1.0y` deleted every snapshot of the backed-up names, the new one
included. 2.1w became one week and 1,5y five years. The fallback now
requires the whole input to be whole-number-and-unit parts with nothing
between them. A bare number is rejected before time.ParseDuration sees
it, since Go reads 0 and +0 as zero with no unit.
Judgement call: a space between number and unit (`30 days`) was
accepted and is now an error, matching Go's own units.
Model: opus-5-5
sneak