Add AGENTS.md and redirect CLAUDE.md there

Tom Hendersonrequested to merge
tomhenderson/ns-3-dev:add-agents-md into master

This is the CLAUDE.md file that I have been using locally. I renamed it to AGENTS.md and redirected Claude to read that (AGENTS.md seems to be the standard file name for this outside of Anthropic).

Please suggest any other tips that you may be using locally to help with agents.

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