Opus finding
Cohort cache age computation uses GNU-only date -d, breaks on macOS/BSD
lowbugmedium
- skills/fork-first-run-alert/SKILL.md:102-110
`date -u -d "..."` is a GNU coreutils extension. On macOS/BSD `date` this fails (the flag means 'set kernel date'). If the skill is ever run on a non-GNU host (or a minimal alpine without coreutils), COHORT_AGE_DAYS stays at 99 and the skill always takes the more expensive live-fallback path — silently degrading correctness/cost rather than failing loudly. This contradicts the documented fast-path policy.
Recommendation
Either document GNU coreutils as a hard requirement, or use a portable form (e.g. `python3 -c` or `gdate` detection, or compute via jq with `now - (fromdateiso8601)`).