Add AGENTS.md with Cursor Cloud development environment instructions

[EXboys]

Summary

• Adds AGENTS.md with Cursor Cloud specific instructions for future agents developing in this codebase.
• Documents non-obvious environment gotchas discovered during setup (RLIMIT_NPROC, python3-venv, non-TTY auto-approve).

Task Linkage

• Task ID: N/A (lightweight environment documentation)

Injected Specs

• spec/architecture-boundaries.md (if architecture/layering changed)
• spec/security-nonnegotiables.md (if sandbox/security changed)
• spec/testing-policy.md (required for any code change)
• spec/docs-sync.md (if behavior/docs/env/commands changed)

Validation Evidence

• Commands executed:
  • cargo build — compiles successfully
  • cargo fmt --check — no formatting issues
  • cargo clippy --all-targets -- -D warnings — clean
  • cargo test — 684 tests passed, 0 failed
  • ruff check python-sdk/ — all checks passed
  • cd python-sdk && pytest — 23 tests passed
  • skilllite run .skills/calculator '{"operation":"add","a":42,"b":58}' — sandbox L3 execution successful
  • skilllite run .skills/text-processor '{"operation":"uppercase","text":"hello world"}' — sandbox L3 with auto-approve successful
• Key results:
  • Full dev environment operational including sandbox (bwrap), security scan, and skill execution
  • Documented that SKILLLITE_MAX_PROCESSES=200 is needed in Cloud Agent VMs

Verification log:
dev_environment_verification.log

Regression Scope

• Areas likely affected:
  • None (documentation-only change)
• Explicit non-goals:
  • No code changes to the application

Docs Sync (EN/ZH)

• Not needed (AGENTS.md is agent-specific tooling, not user docs)

Review Checklist

• No task folder needed (lightweight external/community PR)
• Validation evidence provided above

_{To show artifacts inline, enable in settings.}