[codex] docs: improve MCP client setup README

[however-yir]

Summary

• add an MCP client setup matrix, auth and credential location guidance, and config snippets for Claude Desktop, Cursor, Claude Code, and OpenAI Agents
• replace the flat tool list with a capability matrix and add troubleshooting for long-running media generation flows
• align the example config defaults in .env.example, mcp_server_config_demo.json, and minimax_mcp/__main__.py with the current API host examples

Why

• the README did not clearly explain which client config files to edit, how auth is passed, or how to handle async video generation
• example config files still used an outdated api.minimax.chat host example

Validation

• reviewed the README diff and linked config references
• python3 -m py_compile minimax_mcp/__main__.py
• python3 -m json.tool mcp_server_config_demo.json >/dev/null