Add --group-by team aggregation for org-wide ownership distribution

[raiderrobert]

Problem

An engineering director wants to understand ownership distribution across the org: which teams own the most repos, which own few, and how many repos are unowned. The tool currently reports per-repo, never per-team.

Current workaround

ownrs org acme-corp --format json | jq <post-processing>

Possible approach

A --group-by team flag on ownrs org that aggregates results by team instead of listing per-repo:

$ ownrs org acme-corp --group-by team

Team               Aligned  Mismatched  Stale  Total
platform           35       2           1      38
frontend           22       0           3      25
backend            18       1           0      19
(unowned)          -        -           -      60