Add Mermail MCP plugin

[toannhu96]

Summary

• add Mermail as a productivity MCP plugin backed by the hosted Streamable HTTP endpoint
• configure interactive OAuth without embedding API keys or authorization headers
• include marketplace metadata, installation and safety documentation, changelog, license, and official branding
• register Mermail in the marketplace manifest and root plugin catalog

Validation

• node scripts/validate-plugins.mjs
• upstream Mermail plugin test suite with coverage gates: 93.36% lines, 92.86% branches, 96.43% functions
• duplicate marketplace name and source checks
• manifest path, endpoint, logo, XML, and credential-pattern checks

Notes

This package follows the repository convention for lightweight third-party MCP integrations. The broader Mermail workflow skill bundle remains in the standalone Mermail Cursor plugin repository.