docs(AGENTS.md): refine

[fi3ework]

Summary

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Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[Copilot]

Pull Request Overview

This PR refines documentation by creating package-specific guidelines for create-rslib and restructuring the main repository guidelines for better readability and organization.

• Adds dedicated AGENTS.md file for the create-rslib package with package-specific workflows
• Reorganizes the main AGENTS.md with clearer sections and improved formatting
• Moves architecture details from general guidelines to package-specific documentation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/create-rslib/AGENTS.md	New package-specific documentation covering CLI scaffolding, templates, and development workflows
AGENTS.md	Restructured with better organization, added setup instructions, and moved architecture details to package docs

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