feat: create skill folder about `diffpy` code

[ycexiao]

Problem

Create some command like

diffpy.app agentify system
diffpy.app agentify </path/to/folder>

to deploy skill folder in the workspace or the system ./Claude or ./Codex

Proposed solution

[ycexiao]

@sbillinge, we need to decide two things to implement it:

1. the name for the app. My suggestion is agentify
2. the place to hold the skill folder. My suggestion is https://github.com/Billingegroup/claude-group-standards