docs: clarify verification steps for Claude Code custom-commands tab

[SoYuCry]

Updated "Verify Installation" to mention the custom-commands tab.

Motivation and Context

Plugin commands are located in the custom-commands tab in Claude Code v2+, which isn't the default view. A brief note helps users locate them quickly.

How Has This Been Tested?

Manually verified in Claude Code v2.1.4.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

N/A