refactor: deduplicate optional variable checks

[tomatotomata]

Summary

Extract the duplicated pipe-splitting lookup used for optional target variable groups in CheckVariables into a small groupSatisfied helper. The summary and verbose paths now share the same implementation, preserving the existing behavior while reducing maintenance surface.

Closes #779

Validation

• go test -count=1 ./internal/doctor
• go vet ./internal/doctor
• gofmt -w internal/doctor/doctor.go
• git diff --check
• Divisor council review: Architect, Tester, and Guard all approved.

The repository's full go test -race command could not run locally because this Windows environment has no C compiler for cgo. The change is a pure refactor and the targeted package tests pass.

I used AI assistance for this small refactor and reviewed the repository's AI tooling and contribution guidance.