Problem

Private Sigstore signing (#408) lets an org sign against a self-hosted Fulcio/Rekor, but aicr verify can only verify against the public-good trusted root. verifySigstoreBundle calls trust.GetTrustedMaterial() (pkg/bundler/verifier/trust.go), which has no override for a private CA root or a custom TUF trust root. So privately-signed bundles fail verification.

Proposal

Allow aicr verify to load a custom Sigstore trusted root so bundles signed against private Fulcio/Rekor verify against the matching root of trust.

Behavior

• A flag (e.g. --trust-root <path> / --certificate-chain) or a TUF root URL supplies the private root of trust to the sigstore-go verifier.
• Public-good remains the default when unset.
• Composes with the keyless identity-pinning path (the identity matcher is unchanged; only the root material differs).

Dependencies

• Pairs with: feat(bundle): private Sigstore infrastructure support via --fulcio-url and --rekor-url #408 (private Fulcio/Rekor URLs). This is its verification counterpart.

Success criteria

• A bundle signed via aicr bundle --fulcio-url <private> --rekor-url <private> verifies with the corresponding private trust root.
• Public Sigstore verification is unchanged when no custom root is supplied.
• make test / make lint pass.