Add Cloud Agent development environment config

[ZenulAbidin]

Summary

Adds a .cursor/environment.json so Cursor Cloud Agents get a ready-to-use development environment for zpywallet with zero manual setup.

The project is a pure-Python HD wallet library (no runtime services/daemons), so the environment only needs an install phase — no start/terminals.

What the install command does

• Installs the dev toolchain into the user site (~/.local): python3 -m pip install --user --break-system-packages -r requirements-dev.txt (mirrors the CI step in .github/workflows/commit.yml).
• Installs the package editable (pip install -e .) so import zpywallet works from any directory.
• Idempotently ensures ~/.local/bin (where tox/pytest/flake8 land) is on PATH, so the documented commands in AGENTS.md (tox, python -m pytest ..., tox -e flake8, tox -e docs) work verbatim.

--user/--break-system-packages is used because the Ubuntu 24.04 base image ships an externally-managed (PEP 668) Python; a global sudo pip install collides with Debian-managed packages, and the repo already tracks a committed .venv, so the home-based user site keeps setup clean and repo-path independent.

Validation

Check	Result
tox -e py312 (pytest suite)	164 passed, 7 skipped
tox -e flake8	Passed
tox -e docs (Sphinx -W + rstcheck)	Passed
End-to-end wallet flow (mnemonic → HD xprv vs known vector → bech32 address derivation → DER sign/verify)	Passed
Install idempotence	Ran twice, exit 0, single PATH entry

Notes

• No application code changed. A pre-existing library bug in create_keypair() (a coincurve 21.x API mismatch, unrelated to the environment) was noted but intentionally left out of scope.