Add Cursor Cloud specific instructions to AGENTS.md

[rgilks]

Adds a ## Cursor Cloud specific instructions section to AGENTS.md documenting:

• Node 24 PATH override required because the VM ships Node 22 by default
• wasm-pack v0.14.0 binary installation (v0.15.0 requires Rust 1.91+, above the pinned 1.88.0)
• WebGPU headless caveat (no GPU in cloud VM — 3D renderer errors, SVG/UI still works)
• Quick reference to verification commands

These instructions help future cloud agents set up the dev environment correctly without re-discovering these caveats each session.

Evidence of working environment

App loaded in browser with WebGPU error (expected — no GPU)
Subsector hex map rendered via SVG — data generation and UI working

All checks passing:

• 102 TS unit tests (vitest)
• 83 Rust unit tests (cargo test)
• Rust clippy clean (native target)
• TypeScript typecheck clean
• Rust formatting clean
• Production build succeeds (WASM + Vite)
• Dev server starts and serves at localhost:5173

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