docs: add root AGENTS.md with Cursor Cloud dev environment instructions

[SSC-STUDIO]

Summary

Sets up the Cloud dev environment for Universal Device Toolkit on Linux and documents the durable, non-obvious caveats for future Cloud agents in a new root AGENTS.md.

The VM tooling itself (.NET 10 SDK, Node via nvm, Electron deps) is provisioned through the environment update script/snapshot — this PR only adds the durable operating guide. Root AGENTS.md is force-added because it matches the repo's AGENTS.md gitignore rule, following the same pattern already used for the tracked Plugins/AGENTS.md.

What was set up / verified

• .NET 10 SDK (~/.dotnet) — installed the cross-platform toolchain.
• Cross-platform udt CLI (the runnable Linux app) — builds and runs status, hardware, telemetry, power, profile, plugins, controls, support, doctor, json against real host telemetry.
• Cross-platform test suite — CrossPlatform.Tests: 175/175 passed (Debug + Release build clean).
• Electron UI — npm ci, dev server builds, and the Electron window launches (DISPLAY=:1). npm test: 192/195 passed.

Pre-existing issues found at this commit (not caused by setup, not fixed here)

• Electron npm run typecheck and the renderer fail on missing source modules stores/featuresStore and hooks/useFeature (imported by dashboard-parity components). This blocks the Dashboard from rendering under npm run dev.
• The experimental portable Linux Host does not compile (Host/Rpc/Handlers/GameBoostHandlers.cs references GameBoost*/GameDetection types absent from the UDTWindows=false Lib build).
• npm run lint reports no-undef for Node globals in test files; 3 Electron tests fail (React is not defined in two plugin renderer tests + one order-dependent notification test).

Notes

• Full solution / Host / official plugins remain Windows-only; only the cross-platform surface builds on Linux (matches .github/workflows/linux.yml).
• AGENTS.md documents the key Node-version gotcha: the base image node (/exec-daemon/node v22.14) is too old for the Electron node --test runner, so interactive shells prefer the nvm-managed Node (v22.22) via ~/.bashrc.

  

[SSC-STUDIO]

Skipping: root AGENTS.md is gitignored and already holds the full project rules. The Cloud-only Linux notes are also outdated now that #185 ships a real Linux Host.