skill: create add-new-model and code-review (#26042)

* skill: add new model

* add common pitfalls

* add code review skill

* nits

* add codeowners

* add security review section

* mention about skills in agents.md

* add design review

* exclude ggml-gh-bot

* Apply suggestions from code review

Co-authored-by: Piotr Wilkin (ilintar) <piotr.wilkin@syndatis.com>
Co-authored-by: Xuan-Son Nguyen <thichthat@gmail.com>

* conversion-time weight modification

* nits

---------

Co-authored-by: Piotr Wilkin (ilintar) <piotr.wilkin@syndatis.com>

CODEOWNERS

@@ -120,3 +120,4 @@
 /SECURITY.md                            @ggerganov
 /build-xcframework.sh                   @danbev
 requirements*.txt                       @CISC
+/skills                                 @ngxson