Cosign offline drop (runtime/signals signing)

Version: v2.6.0 (pinned to keep sign-blob arguments aligned with the sprint’s DSSE snippets).
Binary: tools/cosign/cosign → tools/cosign/v2.6.0/cosign-linux-amd64
SHA256: ea5c65f99425d6cfbb5c4b5de5dac035f14d09131c1a0ea7c7fc32eab39364f9
Check: cd tools/cosign/v2.6.0 && sha256sum -c cosign_checksums.txt --ignore-missing
Use: add tools/cosign to PATH or call ./tools/cosign/cosign sign-blob --predicate-type … --output-signature … <file>