📝 docs(playbook): add contributing and multi-language standards

Introduce C++ and Python docs/templates and split .agents rulesets by language (tsl/cpp/python).

Move commit message guidance to CONTRIBUTING.md (full spec remains in docs/common/commit_message.md).

templates/cpp/CMakeLists.txt

@@ -0,0 +1,26 @@
+cmake_minimum_required(VERSION 4.2)
+
+# Enable C++23 Modules + (optional) `import std;` with experimental std module.
+set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "d0edc3af-4c50-42ea-a356-e2862fe7a444")
+
+project(app LANGUAGES C CXX)
+
+set(CMAKE_CXX_SCAN_FOR_MODULES ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_CXX_STANDARD 23)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_MODULE_STD 1)
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
+# Toolchain-specific flags (libc++/runtime/linker/etc) should come from your Conan toolchain/profile.
+
+# Example: entrypoint built from main.cc; modules live in .cppm files.
+add_executable(app main.cc)
+
+# If you use C++ Modules, list module interface units explicitly and keep this in sync:
+#
+# target_sources(app
+#   PRIVATE
+#   FILE_SET cxx_modules TYPE CXX_MODULES FILES
+#     modules/app.cppm
+# )