Adding a skill to help with creating feature flags

[jmatthiesen]

This pull request adds comprehensive implementation guidance for using CloudBees Unify feature flags with the CloudBees Feature Management (ROX) SDK. The new documentation covers best practices for defining, configuring, and evaluating feature flags, with an emphasis on selecting the correct SDK for the project's language and following a universal implementation pattern.

Feature flag implementation guidance:

• Added a detailed SKILL.md for create-feature-flag that instructs developers to use the CloudBees Feature Management (ROX) SDK, not generic alternatives, and to follow the official SDK documentation for the project's detected language.
• Outlined a step-by-step process:
  • Create the flag in Unify and handle errors or permission issues appropriately.
  • Detect the project's language from repository signals instead of assuming a default, and prompt the user if detection is ambiguous.
  • Retrieve and use the exact SDK and setup instructions for the detected language from CloudBees documentation.
  • Implement feature flags using a universal pattern: declare, register, initialize with an environment-linked SDK key, and evaluate using the SDK accessor.
• Included an illustrative Node.js example, with explicit instructions to adapt the pattern to the detected language and not to copy the example verbatim for other languages.