Created plugin scaffold

Property	Value
description	Added the first MAC Codex Codex-plugin manifest, boundary skill, and repo-local context.
version	0.1.0
type	Create
related	MAC Codex, MAC, MAC MCP

• Added the first required .codex-plugin/plugin.json manifest to mac-codex.
• Added a minimal mac-codex boundary skill.
• Added repo-root and repo-local Codex context that keeps shared governance in mac and runtime behavior in mac-mcp.
• Left marketplace publishing, MCP server config, hooks, app integrations, and runtime tools out of this scaffold.