docs: Add Claude Code OAuth plugin warning and troubleshooting

[JoshuaRamirez]

Summary

Adds documentation to help Claude Code users who encounter the "Incompatible auth server: does not support dynamic client registration" error when using the built-in github@claude-plugins-official plugin.

Problem

Claude Code ships with a built-in GitHub plugin that attempts OAuth authentication. However, Claude Code's OAuth implementation requires dynamic client registration, which GitHub's MCP server doesn't support. This causes the plugin to fail silently or with confusing errors, while the documented PAT-based setup works perfectly.

Users often don't realize:

1. The built-in plugin exists and is conflicting
2. The PAT-based manual setup bypasses this issue entirely
3. How to disable the broken plugin

Related issues:

• [BUG] Claude Code does not work with MCP servers that does not implement Dynamic Client Registration anthropics/claude-code#3273
• [BUG] MCP http transport bypasses Authorization header, attempts OAuth2 registration anthropics/claude-code#2831
• Dynamic Client Registration not supported #1404

Changes

1. Added prominent warning at the top of the Claude Code CLI section explaining the OAuth incompatibility
2. Added troubleshooting section with steps to:
   • Disable the conflicting built-in plugin
   • Verify the manual PAT-based setup is working

Why this helps

Many users hit this issue and spend time debugging. This documentation makes the workaround discoverable and saves users from confusion until the OAuth compatibility is fixed upstream in Claude Code.

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🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds documentation to help Claude Code users avoid and troubleshoot OAuth authentication errors when using the built-in GitHub plugin. The documentation explains that Claude Code's built-in github@claude-plugins-official plugin is incompatible with the GitHub MCP server's authentication implementation, and provides clear guidance on using the PAT-based setup instead.

Changes:

• Added prominent warning at the beginning of the Claude Code CLI section about the OAuth incompatibility
• Added new troubleshooting section explaining how to disable the conflicting built-in plugin
• Included steps to verify the manual PAT-based setup is working correctly

[Copilot]

The warning message is very long (over 60 words in a single sentence). Consider breaking it into multiple sentences or bullet points for better readability. For example: Start with the problem statement, then explain the cause, and finally provide the solution in separate sentences.

Suggested change

	> ⚠️ **Important**: Claude Code includes a built-in `github@claude-plugins-official` plugin that attempts OAuth authentication. This plugin currently fails with "Incompatible auth server: does not support dynamic client registration" due to a compatibility issue between Claude Code's OAuth implementation and GitHub's MCP server ([anthropics/claude-code#3273](https://github.com/anthropics/claude-code/issues/3273)). **Use the PAT-based setup below instead**, which bypasses OAuth entirely and works reliably.
	> ⚠️ **Important**
	> Claude Code includes a built-in `github@claude-plugins-official` plugin that attempts OAuth authentication.
	> This plugin currently fails with "Incompatible auth server: does not support dynamic client registration" due to a compatibility issue between Claude Code's OAuth implementation and GitHub's MCP server ([anthropics/claude-code#3273](https://github.com/anthropics/claude-code/issues/3273)).
	> **Use the PAT-based setup below instead.** It bypasses OAuth entirely and works reliably.

[Copilot]

The troubleshooting step mentions setting the value in settings.json under enabledPlugins but doesn't explain the exact JSON structure. Consider adding a brief example showing the complete JSON path, such as: {\"enabledPlugins\": {\"github@claude-plugins-official\": false}} to make it clearer for users unfamiliar with the file format.