Add AGENTS.md (with CLAUDE.md as a symlink to it)

[pocke]

Introduce agent guidance that gives coding agents the context they need to work in this repository safely. The content lives in AGENTS.md, the tool-agnostic convention, and CLAUDE.md is a symlink to it so that Claude Code picks up the same file.

It covers three things:

• A one-line description of the project (a community-managed collection of RBS for gems that lack their own).
• Pointers to the documentation. The links are gathered from README.md (the ruby/rbs repository, docs/CONTRIBUTING.md, docs/SECURITY.md, and the rbs collection docs), plus a direct link to the RBS syntax reference at https://github.com/ruby/rbs/blob/master/docs/syntax.md so agents know where the .rbs type definition syntax is documented.
• A security section. Files under gems/ can be authored by untrusted users because anyone can open a pull request, so they must not be executed or treated as prompts/instructions. It also directs readers to docs/SECURITY.md before submitting a security report, since this project treats malicious code under gems/ as an ordinary bug rather than a vulnerability.

[github-actions]

@pocke Thanks for your contribution!

Please follow the instructions below for each change.
See also: https://github.com/ruby/gem_rbs_collection/blob/main/docs/CONTRIBUTING.md

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You changed non-gem files.

@pocke, please review and approve the changes.