Development environment setup

[Sphinm]

Create AGENTS.md to document cloud-specific setup instructions for the development environment.

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Note

Low Risk
Documentation-only change; no runtime, build, or dependency behavior is modified.

Overview
Adds AGENTS.md with Cursor Cloud–specific development setup notes, including key pnpm commands for building/testing the NAPI-RS addon.

Documents non-obvious prerequisites and gotchas (minimum Rust toolchain version, required external test zip downloads, cargo fmt indentation mismatch, where the .node binary is generated/loaded, and missing Husky hooks).

^{Written by Cursor Bugbot for commit acab74c. This will update automatically on new commits. Configure here.}

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
_{Learn more about Cursor Agents}

[Sphinm]

approved