[codex] Add OAuth MCP config for Codex

[SriganeshNk]

Summary

• Add a plugin-provided .mcp.json entry for the hosted Render MCP server.
• Configure OAuth for Codex with the pre-registered client id codex and the Render MCP resource URL.
• Update Render MCP setup and troubleshooting docs so Codex uses plugin OAuth while manual clients continue to use API-key setup.

Impact

Codex users installing the Render plugin can get the Render MCP server from the plugin manifest and complete OAuth without manually running codex mcp add or setting RENDER_API_KEY for MCP.

Validation

• python3 -m json.tool /Users/nesh/src/render-codex-plugin/.mcp.json
• PYTHONPATH=/Users/nesh/.cache/uv/archive-v0/jKkBJIxRWsYgWH6G/lib/python3.10/site-packages python3 /Users/nesh/.codex/skills/.system/plugin-creator/scripts/validate_plugin.py /Users/nesh/src/render-codex-plugin
• git diff --check

[SriganeshNk]

I still need to verify things and we have security to go through the support because of the dynamically generated path suffix on codex.