Adds cosign install + sign-blob step before commit. The detached
.sig (base64-encoded ASN.1 DER ECDSA over SHA256(file)) is committed
alongside seeded-ci.json. Tower's loader verifies it pure-Go before
replay; mismatched/missing sig → refuse + log.
cosign.pub is also checked in so the workflow can self-verify before
push (catches key-rotation mismatch early). The same pubkey is
embedded in Tower's binary at compat_bootstrap_pubkey.pem; both
copies must match or replay will fail.
