[Bug]: oci export-layout --cosign-key uses the public Rekor log and timestamp authority by default #62

Bug Description

oci export-layout --cosign-key and oci push --cosign-key sign each trust manifest with cosign sign-blob --yes --key <key> --bundle <file> <payload>. Without --signing-config, cosign v3 uses the public Sigstore signing config (signing_config.v0.2.json in sigstore/root-signing), which lists two services that apply when signing with a key:

• Rekor transparency log (https://rekor.sigstore.dev): each signed trust manifest becomes a permanent, public log entry.
• Timestamp authority (https://timestamp.sigstore.dev/api/v1/timestamp): the signature's hash is sent to Sigstore for an RFC 3161 timestamp. Nothing is published, but it is still a third-party request.

--yes skips Cosign's confirmation prompt, and nothing in the CLI output or README mentions either service.

Cosign v3 has no --tlog-upload flag on sign-blob; services are selected through the signing config. With an explicit empty signing config (no Rekor or TSA URLs), the bundle has tlogEntries: 0 and no timestamps. With a config listing only the TSA, it has 0 log entries and 1 RFC 3161 timestamp. I did not run the default path, to avoid creating public log entries.

Expected Behavior

Using either service should be an explicit choice, with offline signing as the default. For example:

• an option selecting the services (log, timestamp, both, or none), plus a way to pass a custom signing config for private deployments, or
• keep the current default but print a clear notice and document it in the README.

Either way, offline signing should be possible without wrapping the cosign binary.

Note that a timestamp is kept only in the Cosign bundle, which the CLI does not store today (#64).

Affected Version

0.3.0, and #61 (aaa5dcb). Observed with cosign v3.1.4.

Steps to Reproduce

COSIGN_PASSWORD= cosign generate-key-pair
COSIGN_PASSWORD= ai-catalog oci export-layout --cosign-key cosign.key --cosign-public-key cosign.pub catalog.json layout

Inspect the bundle Cosign writes during signing for verificationMaterial.tlogEntries and verificationMaterial.timestampVerificationData (running this creates public entries). Workaround: set AI_CATALOG_COSIGN_BIN to a wrapper that adds --signing-config offline.json to sign-blob, where offline.json is:

{"mediaType":"application/vnd.dev.sigstore.signingconfig.v0.2+json","caUrls":[],"oidcUrls":[],"rekorTlogUrls":[],"tsaUrls":[],"rekorTlogConfig":{"selector":"ANY"},"tsaConfig":{"selector":"ANY"}}