docs: add agent skill for datafusion-ffi crate patterns (#22327)

## Which issue does this PR close?

None.

## Rationale for this change

When writing FFI code, there are a variety of established patterns that
it is important to follow. This includes, but is not limited to, things
like ensuring we do not make FFI struct changes on patch releases and
ensuring we are following best practices for checking round trip trait
implementations. The goal of this PR is to add an agent skill to aid
both developers and code reviewers to find issues before they make it
into the code base.

## What changes are included in this PR?

- Add an agent skill.
- Update the AGENTS.md file to explain where to find this skill.

## Are these changes tested?

Yes, I have run the skill and it has already identified gaps in the
current implementation. Issues are opened for current gaps and linked
back to this PR for originating them.

## Are there any user-facing changes?

None

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer