Add Cloud Agent development environment config

[Variiuz]

Summary

Adds a Cloud Agent development environment for CommonwealthMP so agents can build and test the dedicated server end to end on Linux.

• New .cursor/environment.json with an idempotent install that configures and builds the server via CMake.
• .gitignore now tracks .cursor/environment.json while keeping the rest of .cursor/ ignored.

Details

The Linux-buildable surface is the dedicated server plus its test tooling (server/, tools/probe/, tests/). The Windows-only F4SE plugin and the commonlibf4 submodule are not needed for this build, so install skips submodule fetch and stays network-independent.

The default base image already ships CMake 3.28, GCC 13 (C++20), and Git. The default c++/cc is a Clang that resolves the GCC 14 toolchain dir, where the C++ dev libs are absent (cannot find -lstdc++). The install command therefore pins -DCMAKE_CXX_COMPILER=g++, which matches the compiler the Linux CI job effectively uses.

install:

cmake -B build/server -S . -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER=g++ && cmake --build build/server --parallel

No start/terminals are configured because the server is launched on demand during testing rather than as a persistent dev service.

Validation

On the working VM:

• Clean configure + build succeeds; dist/server/CommonwealthMP.Server produced.
• ctest full suite: 33/33 passed.
• Live end-to-end: started the server and connected the probe client; server logged the Hello NEW ... peer=1 host=1 handshake and replied tx Welcome peer=1, probe exited 0.
• install run twice: idempotent (no rebuild on the second run).

In a fresh Cloud Agent booted from a prebuilt environment build of this branch:

• Toolchain present (CMake 3.28.3, g++ 13.3.0, git 2.43.0).
• Install-phase build artifacts already baked in, including /workspace/dist/server/CommonwealthMP.Server.
• Representative test subset 100% green.
• Server + probe Hello -> Welcome handshake completed end to end.

Checklist

• Server and plugin stay on the same protocol (see protocol/)
• Tests updated if the server or protocol changed
• Docs updated if a command, INI key, or join flow changed

No protocol, server, or documented command/flow changed; this PR only adds environment configuration.