docs: add repository contributor guidelines

[toshimaru]

This pull request adds a new documentation file, AGENTS.md, which outlines repository guidelines for contributors. The document provides clear instructions on project structure, build and development commands, coding conventions, testing practices, and expectations for commits and pull requests.

Repository documentation improvements:

• Added AGENTS.md with detailed guidelines on project structure, module organization, and which directories are hand-maintained versus generated.
• Documented build, test, and development commands, including usage of Rake tasks and Jekyll commands for local development and CI.
• Specified coding style and naming conventions for Ruby, JavaScript, YAML, HTML/Liquid, and SCSS files.
• Provided testing guidelines, including minimum validation steps and UI verification instructions.
• Outlined commit message and pull request best practices, including preferred prefixes and requirements for documenting changes.

[Copilot]

Pull request overview

Adds a new contributor-facing documentation file (AGENTS.md) that describes how this repository is structured and how contributors should build, validate, and submit changes for the RailsDoc Jekyll site.

Changes:

• Introduces AGENTS.md describing generated vs hand-maintained directories and the Rails submodule workflow.
• Documents local development/build commands (Bundler, Jekyll, and Rake tasks) plus testing/validation expectations.
• Defines coding conventions and PR/commit hygiene guidance for common file types in this repo.

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