CI policy and `prqnt check`

prqnt check scans and plans without mutating data, then evaluates CI gates.

Example policy

[ci]
fail_on_safe_findings = false
fail_on_review_required = true
fail_on_destructive = true
fail_on_unresolvable_schema = true

Code	Meaning
0	Compliant
2	Repairable safe issues present
3	Review-required operations remain
4	Unresolvable schema conflict or destructive proposal
5	Operational failure

prqnt check dataset/ --policy ci-policy.toml
prqnt check dataset/ --policy ci-policy.toml --json