Clarified MAC Codex runtime boundary

Property	Value
description	Documented that MAC Codex consumes MAC MCP capabilities instead of owning the runtime.
version	0.66.0
type	Update
related	MAC Codex, MAC MCP

• Clarified that MAC Codex owns Codex plugin packaging and client integration.
• Clarified that MAC MCP owns MCP runtime and tool integration implementation.
• Kept the Codex plugin scaffold out of MAC-198.