test(exposure): cover shipped threat intel catalogs

[vincent-peng]

Summary

• add regression coverage that every shipped threat_intel catalog loads individually and via directory mode
• assert shipped catalog entries use supported ecosystems and are non-empty
• assert threat_intel/README.md mentions each shipped catalog
• update README selftest example from 2 to 3 findings to match current fixtures

Verification

• go test -count=1 ./...
• go test -race -count=1 ./...
• go vet ./...
• gofmt -l .
• go build -buildvcs=false ./cmd/bumblebee
• ./bumblebee selftest
• git diff --check HEAD~1 HEAD