Set up Cursor Cloud dev environment for net4cpp

[ruoka]

Summary

Sets up the development environment for net4cpp (a C++23 modules networking/HTTP library) in Cursor Cloud and documents the non-obvious startup caveats for future agents.

What was done

• Installed the LLVM 21 toolchain (clang-21, libc++-21-dev, libc++abi-21-dev, lld-21, clang-tools-21, clang-tidy-21, cppcheck, libssl-dev) from apt.llvm.org — only clang 18 was present. Baked into the VM image.
• Initialized the deps/tester git submodule (test framework + build core; the project's only external dependency).
• Added AGENTS.md with a ## Cursor Cloud specific instructions section capturing the key gotcha (see below).

Update script

git submodule update --init --depth 1 deps/tester

Minimal, idempotent dependency refresh. The LLVM toolchain lives in the VM image, so it is not reinstalled per session.

Verified working

Task	Command	Result
Build	./tools/CB.sh debug build	SUCCESS
Test	NET_DISABLE_NETWORK_TESTS=1 ./tools/CB.sh debug test	393/393 tests, 881/881 assertions pass
Lint	cppcheck ... net/ tools/ + clang-tidy-21	runs (non-blocking, matches CI)
Hello-world	echo server + client over IPv6 loopback (net::acceptor / net::connect)	Hello, net4cpp! echoed back

Key gotcha (documented in AGENTS.md)

tools/CB.sh only auto-disables network tests when CURSOR_SANDBOX is set, which it is not in this cloud environment. So network tests run by default and a few fail against the restricted sandbox network. Use NET_DISABLE_NETWORK_TESTS=1 for a fully-green run. Loopback networking works, so echo/HTTP demos run fine.