Add AGENTS.md with Cursor Cloud dev environment instructions

[EXboys]

Summary

Adds AGENTS.md with Cursor Cloud specific instructions for future agents, covering:

• How to run lint/test/build (mirrors CI pipeline)
• Key gotchas for Cloud Agent VMs (sandbox namespace limitations, python3-venv requirement, PATH setup)
• Quick reference for running skills with appropriate env vars

Validation

All CI-equivalent checks pass in the Cloud Agent environment:

• cargo fmt --check — pass
• cargo clippy --all-targets -- -D warnings — pass
• cargo test — 567 tests pass
• cargo deny check bans — pass
• Python SDK: ruff check, ruff format --check, mypy, pytest (23 tests) — all pass
• Application runs: skilllite --version (0.1.29), skilllite list (14 skills), skill execution verified

dev_env_verification.log

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