Owner activity status in risk output

git-arch risk now shows when the dominant owner of a flagged scope
last committed anywhere in the repo.

62% concentration with an owner active 3 days ago and 62%
concentration with an owner inactive for 2 years are very
different risk stories — same number, different meaning.

When the gap exceeds 18 months, the reason field notes it directly.

npm install -g git-archaeologist
git-arch risk /path/to/repo