chore: use native Codex MCP config instead of mcp-remote

[saif-at-scalekit]

Summary

• Update all 5 plugin .mcp.json files to use Codex native HTTP MCP config instead of the mcp-remote stdio proxy.
• Keep the Scalekit MCP URL normalized with a trailing slash.
• Add Codex troubleshooting notes: run codex mcp login scalekit after install if tools are missing, then restart Codex.
• Clarify that Codex may still display Auth: Unsupported; Tools: (none) is the actionable failure signal.

Before

{
  "mcpServers": {
    "scalekit": {
      "command": "npx",
      "args": ["-y", "mcp-remote", "https://mcp.scalekit.com"]
    }
  }
}

After

{
  "mcpServers": {
    "scalekit": {
      "type": "http",
      "url": "https://mcp.scalekit.com/"
    }
  }
}

Verification

• python3 scripts/validate_marketplace.py
• git diff --check
• Verified codex mcp login scalekit succeeds locally and tools become accessible after authentication.