Add a capability

Define the shared contract once. Implement its handler where the data and infrastructure live. Do not write separate business logic for each interface.

1. Define the contract

Put cross-process contracts in packages/core/src/contracts.ts and import defineContract from @rat-stack/capability/contract.

1. Define the input, output, and expected failure schemas.
2. Give the contract a stable name and short description.
3. Use Schema.Struct for the input.
4. Set honest annotations such as readOnly, idempotent, destructive, and openWorld.
5. Set needsApproval: true when the action needs approval. implement adds the Approval requirement and ApprovalDenied failure; the handler itself declares only the contract's failures.