Add AGENTS.md and .agents/skills instruction generation

[begelundmuller]

Contributes to https://linear.app/rilldata/issue/PLAT-425/documentationimprovement-for-ai-skills

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!