Point MCP server at hosted remote (mcp.resend.com)

[dielduarte]

What

Switch the bundled MCP server from the local stdio server (npx resend-mcp) to Resend's hosted remote MCP at https://mcp.resend.com (streamable HTTP), in both .mcp.json and the duplicate root mcp.json.

{
  "mcpServers": {
    "resend": {
      "type": "http",
      "url": "https://mcp.resend.com",
      "headers": { "Authorization": "Bearer ${RESEND_API_KEY}" }
    }
  }
}

Auth is unchanged — same RESEND_API_KEY, now passed as a bearer header instead of an env var to a local process. Only the transport moves (local → remote); no new credential, no OAuth flow.

Applies across all four platforms: Claude Code (auto-discovers .mcp.json), OpenAI Codex (.codex-plugin/plugin.json references ./.mcp.json), Cursor, and Grok.

README "MCP Server" section updated to describe the hosted endpoint + RESEND_API_KEY.

Follow-ups (not in this PR)

• After merge, bump the pinned sha in xai-org/plugin-marketplace#32 to this commit — that clears the temporary stdio regression in the Grok catalog (it'll read http again).
• Switching the server default to OAuth (dropping the bearer header, documenting API-key as an opt-in override) is a separate later PR.