Design artifact adapter registry

[codeforester]

Summary

• Add an artifact adapter registry design document.
• Define a phased path from a declarative built-in registry to optional workspace overlays while deferring repo-local registry support.
• Clarify schema, adapter contracts, command behavior, diagnostics, migration guidance, and non-goals.

Validation

• git diff --check

Demo Impact

• None. Design/documentation-only.

AI Context

• Not applicable. Design document only; no current command behavior changes.

Closes #816