ci: publish the registry entry alongside the npm release

Publishing the registry entry by hand means it silently goes stale the
first time someone ships a version without remembering to run
mcp-publisher, and a registry that advertises a version npm does not
serve is worse than no entry at all.

The step runs after npm publish because the registry stores only
metadata and validates against the live artifact. It hard-fails when
server.json and package.json disagree on the version, so the two cannot
drift apart unnoticed.

Authentication is the DNS key rather than GitHub OIDC: OIDC only grants
io.github.* namespaces, and this entry lives under dev.useaxis, verified
by the TXT record on the domain.

The download URL and tar invocation were run verbatim before committing;
the release asset carries no version in its filename.