📝 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/.clangd

@@ -0,0 +1,25 @@
+CompileFlags:
+  # Point clangd to a CMake compilation database (compile_commands.json).
+  #
+  # Recommended: keep this aligned with your Conan/CMake preset output directory.
+  # Example (this playbook's default cross build folder):
+  CompilationDatabase: build/windows-x86_64-clang-cross/Release
+
+---
+If:
+  PathMatch: [.*\\.hpp, .*\\.hxx, .*\\.cpp, .*\\.cc, .*\\.cxx, .*\\.cppm, .*\\.ixx, .*\\.mpp]
+
+CompileFlags:
+  Add:
+    - "-std=c++23"
+  Compiler: clang++
+
+---
+If:
+  PathMatch: [.*\\.h, .*\\.c]
+
+CompileFlags:
+  Add:
+    - "-std=c17"
+  Compiler: clang
+