docs(agents): add cpp-code-review skill

Add a code review skill with reference files for memory safety,
concurrency, correctness idioms, false positives, cppreference
lookup, and report format. Move the coding conventions from
AGENTS.md into the skill's project-style reference so reviews
enforce them.

Co-authored-by: Qwen 3.8 27B <qwen3.8_27b-q3_k_xl@lmstudio.local>

.agents/skills/cpp-code-review/references/project-style.md

@@ -0,0 +1,31 @@
+# Project Conventions
+
+## Coding Conventions
+
+- **Language:** C++23
+- **Primitive types:** use fixed-width type aliases (`u32`, `i64`, ...) instead
+  of `std::uint32_t` etc.
+- **Trailing return type** for function signatures (e.g. `auto fn() -> void`)
+- **4-space indentation**
+- **No semicolons after closing braces** for namespaces/classes
+- `auto` for obvious types (e.g. `auto main(...) -> int`)
+- **East const** (e.g. `char const*` not `const char*`)
+- **Trailing return type** for all function definitions, including operators
+  (e.g. `auto operator=(T&&) noexcept -> T&`)
+- **Public members first** in class declarations, private members at the bottom
+- **Private data members** start with `m_` (e.g. `m_state`)
+- `<>` includes only for system headers (std, OS, etc.)
+- `""` includes for third-party dependencies (e.g. `fmt`, `nlohmann/json`)
+- **Naming:** `snake_case` for variables, functions, and classes
+- **Naming:** `SCREAMING_SNAKE_CASE` only for macros and constants
+- **Header extensions:** C++ headers use `.hpp`, C headers use `.h`
+- Include order:
+  1. C++ standard library headers (`<chrono>`, `<vector>`, etc.)
+  2. _(blank line)_
+  3. C standard library headers (`<stdlib.h>`, `<string.h>`, etc.)
+  4. _(blank line)_
+  5. OS-specific headers (Windows API, POSIX, etc.)
+  6. _(blank line)_
+  7. Third-party dependencies (`"fmt/core.h"`, etc.)
+  8. _(blank line)_
+  9. Local/project headers