fix(core): correct MCP environment field in built-in skill

[OpeOginni]

Issue for this PR

Closes#39135

Type of change

• Documentation

What does this PR do?

• Corrects the MCP examples in the customize-opencode skill to use environment rather than the unsupported env field.
• Documents that environment supplies variables to local MCP server processes.
• This matches the active MCP configuration schema, preventing agents from generating invalid configuration.

How did you verify your code works?

• git diff --check
• bun turbo typecheck via the repository pre-push hook

Screenshots / recordings

• Not applicable: documentation-only change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search results, I found related PRs that address similar MCP environment field issues:

1. PR fix: use 'environment' key for MCP env vars in skill example #38969 - "fix: use 'environment' key for MCP env vars in skill example"

   • Directly related to the same MCP environment field correction

2. PR fix(skill): correct MCP local server env key to environment in customize-opencode #35867 - "fix(skill): correct MCP local server env key to environment in customize-opencode"

   • Addresses the same env → environment field correction in the customize-opencode skill

These PRs appear to be addressing the same or very similar MCP configuration issues. You may want to verify if PR #39175 is indeed a new/separate effort or if it should be consolidated with the existing work in these PRs.