Add AGENTS.md with Cursor Cloud development environment setup instructions

[hzsw1234]

Summary

Adds AGENTS.md with Cursor Cloud specific development instructions for the cursor-api project. This file documents:

• Required system dependencies (Rust nightly, protoc, Node.js/npm, libssl-dev)
• Build, lint, test, and run commands
• Environment variable requirements (AUTH_TOKEN, PORT, etc.)
• Known pre-existing test issues
• npm dependencies for build-time asset minification

Verified

• cargo build (dev profile) - successful
• cargo clippy - warnings only (no errors)
• cargo test - 25 tests found; 1 pre-existing abort in test_build_routes_deduplication
• Server runs on port 3000 with health check, model listing, and utility endpoints all functional