Add Cloud Agent development environment config

[kumagi]

Summary

Adds a Cloud Agent development environment configuration (.cursor/environment.json) for tinylamb, a C++20 RDBMS built with CMake.

The environment uses Cursor's default image plus an idempotent install step that installs the toolchain and tools the project needs:

• build-essential + libstdc++-14-dev — C++20 toolchain and the libstdc++ dev files clang selects (the default image ships libgcc-14-dev but not libstdc++-14-dev, which otherwise breaks linking with cannot find -lstdc++).
• cmake, ninja-build, clang — the build system and compiler used by CI and the README.
• clang-format — required by the clang-format CI check.
• postgresql-client — psql for exercising the PostgreSQL-wire tinylamb_server.
• curl, git, ca-certificates — for the pinned GoogleSQL / googletest / TPC-H DBGEN downloads CMake performs.

The build downloads its dependencies (pinned GoogleSQL execute_query binary with SHA-256 verification, googletest v1.14.0 via FetchContent, and the TPC-H DBGEN source) over the network, so no vendored deps are required.

Validation

• Toolchain chain proven end-to-end on the repo's real code: a clean clang + CMake + Ninja + googletest FetchContent build compiled parser/tokenizer.cpp and ran parser/tokenizer_test.cpp under ctest — all 6 cases passed.
• The pinned GoogleSQL frontend binary downloaded (checksum-verified) and executed a real query.
• install is idempotent (second run reports 0 added, 0 removed).

Note on repository build state

The full project does not build at the current master (d37c659): CMakeLists.txt and README.md reference roughly 60 source files that are not committed to the repository — the entire server/ and benchmark/ directories plus query/sql_engine.cpp, query/googlesql_*.cpp, several executor/, expression/, and plan/ sources. CMake configuration therefore fails before any target can be built. This is a codebase issue independent of the environment; once those sources are committed, the configured environment builds and tests the project with the standard cmake -S . -B build -G Ninja && cmake --build build -j && ctest --test-dir build flow.