Rework agent code review guidance (#31969)
### Description
<!-- Describe your changes. -->
Try to organize agent guidance so it is more extensible. E.g., add
path-specific instructions, code review skill.
Also moved skills to `.github/skills`. Added pointer to this location in
AGENTS.md.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Goal is to improve agent code reviewing by providing guidance to follow.
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
