Add Cursor Cloud specific instructions to AGENTS.md

[e-fu]

Summary

Adds a ## Cursor Cloud specific instructions section to AGENTS.md documenting the development environment setup for Cursor Cloud agents.

What was added

• Erlang/OTP 27 + Elixir 1.18.4 runtime paths and installation method
• Note that precompiled NIFs (QuickBEAM/Zig, OXC/Rust) download automatically — no native toolchains needed
• Reference to standard test/lint/build commands (from CLAUDE.md/README.md)
• Known compilation warnings (dialyzer_json Elixir version, descripex Jason warnings)

Verification

All checks pass on this environment:

• mix test — 1 test, 0 failures
• mix format --check-formatted — clean
• mix credo --strict — no issues
• mix compile — successful (with expected warnings)
• QuickBEAM JS runtime — eval, call, async, vars all working

env_setup_verification.log

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