Fix marcomoauro/substack-mcp setup instructions

[marcomoauro]

Summary

• replace the nonexistent @marcomoauro/substack-mcp package with the published substack-mcp@latest
• document the three required session credentials and safe browser retrieval steps
• fix the Claude Code command/config format and the Node.js 22 requirement
• align the feature comparison with the server's current 27 tools, including image uploads and analytics, while removing unsupported Note creation and scheduling claims

Verification

• parsed every changed JSON example and templates/mcp-config.json
• launched npx -y substack-mcp@latest with placeholder credentials on Node.js 22
• checked that obsolete package, environment-variable, and config-path references are gone