Problem

KMS-backed signing (#407) lets CI sign a bundle with a cloud KMS key, but aicr verify cannot verify the result. The verifier (pkg/bundler/verifier/verifier.go → verifySigstoreBundle) only verifies keyless Sigstore bundles against the public-good trusted root (trust.GetTrustedMaterial()); there is no public-key verification path.

Proposal

Add a --key <kms-uri-or-pubkey> option to aicr verify that verifies a key-signed bundle against the corresponding public key (KMS URI or a local PEM), mirroring cosign verify --key.

Behavior

• --key selects public-key verification instead of keyless certificate-identity verification.
• Accepts the same KMS URI schemes as signing (awskms://, gcpkms://, azurekms://, hashivault://) and local public-key files.
• Mutually exclusive with the keyless identity-pinning path (--certificate-identity-regexp).

Dependencies

• Pairs with: feat(bundle): KMS-backed signing for CI/CD environments without OIDC #407 (KMS-backed signing). This is its verification counterpart.

Success criteria

• A bundle signed with aicr bundle --signing-key gcpkms://... verifies with aicr verify --key gcpkms://....
• make test / make lint pass.