Implement attested CI publishing for registry plugins

[ascorbic]

Goal

Plugin authors should be able to publish releases from CI with verifiable provenance, as described in the registry RFC.

Scope

Add the CLI, registry record, and aggregator support needed to publish from CI without treating a long-lived publisher secret as the trust model.

Acceptance criteria

• CI publishing binds a release to publisher identity and workflow provenance.
• Published records expose enough metadata for the aggregator/admin to display or audit provenance.
• Artifact checksum and capability manifest validation still apply.
• Documentation explains setup and the trust model.
• Tests cover accepted and rejected attested publishing flows.