docs: add ${CLAUDE_PLUGIN_ROOT} explanation to marketplace-structure skill (#97)

## Summary

Add explanatory note for the `${CLAUDE_PLUGIN_ROOT}` environment
variable in the marketplace-structure skill's schema reference
documentation.

## Problem

Fixes #95

The `${CLAUDE_PLUGIN_ROOT}` environment variable is used in the hooks
and mcpServers examples within the Complete Plugin Entry Example, but
wasn't explicitly explained. Users unfamiliar with plugin development
may not understand what this variable represents.

## Solution

Added a brief note directly before the Complete Plugin Entry Example
that explains:
- What `${CLAUDE_PLUGIN_ROOT}` resolves to (plugin's absolute
installation path)
- When to use it (hooks and MCP server configurations)
- Why it matters (portable paths regardless of installation location)

This approach was chosen over adding a new subsection because:
1. It's contextual - explains the variable right where it's used
2. Simpler and less intrusive to the document structure
3. Provides just-in-time information for users reading the example

### Alternatives Considered

- **New "### Environment Variables" subsection** after Component
Configuration Fields - More prominent but disrupts document flow and may
be overkill for a single variable note
- **Inline code comment** in the JSON example - Would require multiple
comments and break JSON syntax highlighting

## Changes

-
`plugins/plugin-dev/skills/marketplace-structure/references/schema-reference.md`:
Added note before Complete Plugin Entry Example explaining
`${CLAUDE_PLUGIN_ROOT}`

## Testing

- [x] markdownlint passes
- [x] No other files affected

---
🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: sjnims

plugins/plugin-dev/skills/marketplace-structure/references/schema-reference.md

@@ -150,6 +150,8 @@ For GitLab, Bitbucket, or self-hosted git repositories:
 
 ## Complete Plugin Entry Example
 
+**Note:** The `${CLAUDE_PLUGIN_ROOT}` variable in the example below resolves to the plugin's absolute installation path. Use this variable for portable paths in hooks and MCP server configurations, ensuring paths work regardless of where the plugin is installed.
+
 Advanced plugin entry with all optional fields:
 
 ```json