skill: create add-new-model and code-review

[ngxson]

Overview

Add skills for AI coding.

While this doesn't avoid low-effort slop contributions, it will hopefully reduce the review effort by guiding AI to write better code from the beginning.

2 skills are added:

• add-new-model: can be viewed as an extension of HOWTO-add-model.md with more detailed guidance
• code-review: per-component review skill, currently support src/models/, server and GGML changes

How these skills are made?

• I draft a list of requirements / common pitfalls
• I asked the AI to read N recent PRs to confirm & refine that list
• Then finally write the SKILL.md
• I review the output manually and made some adjustments

CC @ggml-org/maintainers for review & discussions

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: yes

[ngxson]

I've been testing the code-review skill on some recent PRs and it shows very promising results, mostly aligned with project's convention.

Will experiment with adding it to @ggml-gh-bot and allow maintainers to trigger it manually.

[ngxson]

I kinda need the code-review skill now, is this good to merge @ggml-org/maintainers ?

Will experiment with @ggml-gh-bot review command next, but that will be my low-priority task. For now it's still quite helpful to ask pi / claude to "review PR ... using code-review skill", that works quite well