chore(ci): pin mcp-publisher >=1.7.6 and make publish steps idempotent

[alexpota]

What changed

.github/workflows/publish.yml only.

1. mcp-publisher v1.5.0 → v1.7.9 (floor v1.7.6 — OIDC audience fix from modelcontextprotocol/registry PR #1229). EXPECTED_SHA updated to match.
2. npm publish step skips if the version is already on npm.
3. MCP Registry publish step skips if the version is already on the registry (via /v0/servers/<name>/versions).
4. workflow_dispatch trigger added with required tag input. Checkout uses github.event.release.tag_name || inputs.tag so a manual dispatch builds the requested tag.