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/correctness-idioms.md

@@ -0,0 +1,41 @@
+# Correctness & Modern Idioms
+
+Docs: cppreference language/library pages; C++ Core Guidelines.
+
+## Errors & exceptions
+
+- Errors aren't silently ignored (unchecked return codes, empty `catch (...)`).
+- Mark functions whose result must be used `[[nodiscard]]`.
+- Exception safety: on throw, no leaks and objects stay valid (RAII gives this
+  for free).
+- Destructors and move operations are `noexcept`; no throwing from destructors.
+- Catch exceptions by const reference (`catch (std::exception const& e)`).
+- Consider `std::optional` / `std::expected` (C++23) for expected failures, per
+  project style.
+
+## Types & integers
+
+- Fixed-width integers: use the project's aliases (`u32`, `i64`, ...) instead of
+  `std::uint32_t` etc., e.g. for sizes in protocols, files, and hardware.
+- Signed/unsigned comparison and mixing (`int` vs `size_t`); use `std::cmp_less`
+  etc. (C++20) or `std::ssize`.
+- Narrowing conversions are explicit and checked; prefer `{}` initialization to
+  catch them.
+- No implicit conversions via single-arg constructors; mark them `explicit`.
+- `enum class` over plain `enum`.
+
+## API & classes
+
+- `const` correctness: methods, references, and parameters.
+- Pass cheap types by value, large read-only by `const&`, sinks by value +
+  `std::move`.
+- `override` on virtual overrides; `final` where intended.
+- No object slicing (passing derived by value as base).
+- Header hygiene: include what you use, no `using namespace` in headers.
+
+## Modern idioms
+
+- Prefer standard algorithms and range-for over hand-rolled loops.
+- `constexpr` for compile-time constants instead of macros.
+- `std::string_view` / `std::span` for non-owning parameters (watch lifetimes).
+- New logic has tests; edge cases (empty, max, error path) are covered.