claude: add "skill" to assist with FilterX LLVM IR code generation

[MrAnno]

Do not merge this, but feel free to use it.

2 issues with this PR that prevents it from merging:

1. I'm really reluctant to add such "skill" documents to the AxoSyslog repo, as these documents are plain English markdown files. Why would we write such "guides" for LLMs only and place them inside a directory called .claude if we could write the same documentation for humans and publish it as a normal developer guide?
2. The instructions and practices inside this document are temporary, we'll have to change approaches and a lot of things once we are done writing the initial compile() methods for all expressions.

Problem 1. can be resolved by writing a maintainable guide for developers and symlinking each section as a separate Claude skill.