Catalog dataset guidelines and add OpenHands PR review

[neubig]

Summary

• Expand AGENTS.md with strict dataset incorporation guidelines, including generator-first sample creation, schema/API/SFT validation expectations, common prior-PR failure modes, and a required design-decision catalog for unclear choices.
• Add the OpenHands PR review workflow from OpenHands/extensions so PRs can be reviewed automatically or on demand.
• Add repository-specific /codereview guidance that instructs the review agent to read AGENTS.md and verify the dataset guidelines and PR-description requirements.

Testing

• git --no-pager diff --check
• Python marker/trailing-newline validation for AGENTS.md, .agents/skills/custom-codereview-guide.md, and .github/workflows/pr-review-by-openhands.yml
• No application-code tests were added or run because this change only updates documentation and GitHub/workflow configuration.

Fixes #185

This PR was created by an AI agent (OpenHands) on behalf of the user.

@neubig can click here to continue refining the PR