v1.5.1

fix(registry): re-list under io.github.unsoldgroup — v1.5.1 (#30)

The old MCP Registry namespace was unreachable, not just misconfigured.
`io.github.alunsoldantarctica/*` derives from a GitHub username that no
longer exists (the account was renamed, the org rename followed), and the
registry grants a namespace only to a live identity — so nobody could
authenticate to it: not CI's OIDC identity, not a browser login, not the
original owner. Both attempts 403'd during the v1.5.0 release.

- server.json `name` and package.json `mcpName` -> io.github.unsoldgroup/
  mail-index. mcpName is the registry's npm ownership check, so the rename
  only takes effect via a published version: 1.5.1 in all four version
  fields.
- registry-publish.yml: the `release: [published]` trigger NEVER fires for a
  Release created by release.yml — GitHub refuses to start a workflow from an
  event another workflow raised with the default GITHUB_TOKEN. Documented in
  the workflow and in PUBLISHING.md; the step is a manual dispatch.
- PUBLISHING.md prerequisites, verification URLs, and the 403 troubleshooting
  note rewritten for the org namespace.

The npm package is still `mail-index`; installs, MCP configs, and the tool
surface are unchanged. Only the registry listing's identifier moved. The old
entry stays frozen at 1.0.0 permanently.

Co-authored-by: al-unsoldgroup <al-unsoldgroup@users.noreply.github.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>