Atrium is the economic and identity layer beneath existing agent runtimes, not a new runtime. A skill is to an AI agent what an npm package is to a Node program, but signed by its author, addressed by its content, and paid per call. AI agent runtimes increasingly generate reusable skills, but those skills are trapped inside the runtime that produced them — no portable record of authorship, no way to verify what a consumer is about to run, and no mechanism to compensate authors when their work is reused. This paper specifies the manifest format, the on-chain settlement contract and its conservation invariants, the royalty model, the threat model, and the off-chain architecture, and argues that a tokenless, per-invocation design is the right primitive for an emerging skill economy.