Add skill about 2 steps method from Kent Beck -> 051-design-two-steps-methods

[jabrena]

Issue title:
Add skill 051-design-two-steps-methods for Kent Beck’s two-step change method

Issue body:

User Story

As a Java Enterprise development practitioner using this rules and skills collection
I want to have a dedicated skill named 051-design-two-steps-methods that explains and applies Kent Beck’s principle, “Make the change easy, then make the easy change”
So that I can approach complex code changes through preparatory refactoring before applying the intended behavior change.

Context

Create a new skill covering Kent Beck’s two-step method:

“Make the change easy, then make the easy change.”

Reference: https://x.com/KentBeck/status/250733358307500032

Acceptance Criteria

Feature: Design changes using the two-step method

@acceptance-test
Scenario: Apply Kent Beck's two-step method to a code change
  Given a developer needs to make a complex or risky code change
  When they use the 051-design-two-steps-methods skill
  Then the skill guides them to first make the change easy through preparatory refactoring
  And then make the intended behavior change once the design supports it

Notes

The skill should be authored from the XML sources under skills-generator/src/main/resources/, not by editing generated output under skills/.