Document the built-in Agent Skill in the README

[sander1095]

Document Mockly's Agent Skill in the README

Mockly already ships an Agent Skill and copies it into .agents/skills/mockly, but that is easy to miss today.

Adding a short README note would make this feature easier to discover and document the automatic copy behavior plus the opt-out via <MocklySkill>false</MocklySkill>.

Related work

• Add Mockly Copilot skill and auto-copy to .agents/skills #108 — Add Mockly Copilot skill and auto-copy to .agents/skills
• Add Mockly Copilot skill #138 — Add Mockly Copilot skill
• Ensure copied Mockly skill content is ignored in consuming repos #141 — Ensure copied Mockly skill content is ignored in consuming repos