docs: add Cursor Cloud dev environment instructions

[zheli]

Summary

Sets up and documents the development environment for this canton-devkit distribution/packaging repository. This repo has no build step, no dependency manifest, and no automated test suite — it ships the canton-devkit binary via an installer script, a Homebrew formula, and a static APT repo.

Adds AGENTS.md with a ## Cursor Cloud specific instructions section describing the dev tooling and how to lint/validate/run each piece. No existing code is modified.

Dev environment

• Installed shellcheck (lint install.sh) and ruby (syntax-check the formula). dpkg/dpkg-scanpackages are already in the base image.

Verification

Component	Command	Result
Installer lint	shellcheck install.sh	Only SC2016 info (intentional literal $PATH)
Homebrew formula	ruby -c Formula/canton-devkit.rb	Syntax OK
APT repo index	dpkg-scanpackages --multiversion pool /dev/null vs committed Packages	Reproducible / identical
APT Release checksums	sha256sum of Packages/Packages.gz	Match
Hello world	sudo dpkg -i .../canton-devkit_0.16.3_amd64.deb then canton-devkit localnet --help	Prints Canton LocalNet CLI

Full run log is attached as the dev_env_demo.log artifact.

Known caveat (pre-existing, not fixed here)

install.sh downloads checksums.txt, but every published release ships the checksum file as SHA256SUMS, so install.sh fails at the checksum step against real releases. Documented in AGENTS.md; left unchanged per "do not modify existing code".