Add create-pull-request skill

[Raghaddahi]

Fixes #437

Description

• Add a standardized agent skill for creating pull requests to ensure consistent PR quality, process, and AI usage disclosure across the project.
• This is a new Markdown skill file with no runtime code changes.
• No additional QA steps required beyond reviewing the skill content for accuracy and completeness.

AI usage

• Agent: AI assisted with PR description only. This code was reviewed and verified by a human.