Add AGENTS.md with Cursor Cloud development environment instructions

[GUOF629]

Summary

Adds AGENTS.md with Cursor Cloud specific development instructions for the opcode codebase.

What's included

• Overview of the project architecture (Tauri 2 + React + Rust)
• Key development commands reference table
• Important caveats for Cloud Agent environments:
  • Frontend must be built before Rust compilation (dist/ dependency)
  • Headless mode instructions using opcode-web binary
  • Bun as primary package manager with PATH setup
  • System dependencies required on Linux
  • SQLite bundled (no external DB needed)
  • Claude Code CLI optional for development
  • Expected clippy warnings explanation

Development environment verified

• Lint: bunx tsc --noEmit, cargo check, cargo clippy, cargo fmt --check all pass
• Tests: cargo test - 9/9 tests pass
• Build: Frontend (vite build) and Rust backend (cargo build --bin opcode-web) both build successfully
• Run: Web server mode running on port 8080, serving the full React frontend with working API endpoints