git-arch risk: maintenance risk map

New command: git-arch risk

Reuses the existing bus factor, ownership concentration, and
contributor count data — no new analysis — and reframes it as
LOW/MEDIUM/HIGH risk areas with plain-language explanations.

Example, from Express:

HIGH RISK
support
Bus Factor: 1 Ownership Concentration: 100% Contributors: 1
Reason: Single dominant maintainer (Douglas Christopher Wilson)
with limited contributor redundancy.

Also fixes a small inconsistency — the risk command now applies
the same bot-filtering as bus factor analysis, so the two stay
in sync.

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