Support diffing against a previous run to detect ownership changes

[raiderrobert]

Problem

After a bulk ownership migration (e.g., adding catalog-info.yaml to 50 repos via script), the user wants to verify the migration worked by comparing before and after states. Currently the tool only shows a point-in-time snapshot — the user has to manually save and compare JSON outputs.

Current workaround

# before migration
ownrs org acme-corp --team my-team --format json > before.json

# run migration script

# after migration
ownrs org acme-corp --team my-team --format json > after.json

# manually diff
diff before.json after.json

Possible approach

A --diff <FILE> flag that compares current results against a previous JSON output and reports what changed:

$ ownrs org acme-corp --team my-team --diff before.json

Changed: 48 repos
  codeowners-only → aligned:  45
  codeowners-only → mismatched: 2
  missing → catalog-only: 1
Unchanged: 2 repos

This would also be useful for tracking ownership health over time (weekly runs with diffs).