Add ofrak-developer Claude skill

[rbs-afflitto]

• [x] I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.
• [x] I have made or updated a changelog entry for the changes in this pull request.

One sentence summary of this PR (This should go in the CHANGELOG!)

Adds the ofrak-developer Claude Skill for LLM assisted OFRAK development.

Link to Related Issue(s)

N/A

Please describe the changes in your request.

Adds the ofrak-developer Claude Skill for LLM assisted OFRAK development. Claude Skills gives Claude Code additional instructions and documentation for working on a specific task in a packageable format. The ofrak-developer skill provides Claude with sufficient context about OFRAK and its documentation to be able to make more meaningful and correct changes to OFRAK compared to using Claude on it's own.

Anyone you think should look at this, specifically?

@whyitfor