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/cppreference.md

@@ -0,0 +1,40 @@
+# cppreference Lookup
+
+Use cppreference to verify rules and to link in findings. Fetch pages with web
+fetch when unsure.
+
+## How to look things up
+
+- Search any symbol, header, or keyword:
+  `https://cppreference.com/index.php?title=Special%3ASearch&search=<TERM>` e.g.
+  `...&search=cstdint`, `...&search=std%3A%3Aspan`, `...&search=memory_order`
+  The results page links to C++ and C entries separately; pick the right column.
+- Each page lists which standard (C++11/17/20/23) added a feature; check it
+  against the project's `-std`.
+
+## Entry points
+
+- Language: https://en.cppreference.com/cpp/language
+- Standard library headers: https://en.cppreference.com/cpp/header
+- Fixed-width integers `<cstdint>`:
+  https://en.cppreference.com/cpp/header/cstdint
+- Undefined behavior: https://en.cppreference.com/cpp/language/ub
+- Object lifetime: https://en.cppreference.com/cpp/language/lifetime
+- Initialization: https://en.cppreference.com/cpp/language/initialization
+- Evaluation order: https://en.cppreference.com/cpp/language/eval_order
+- Exceptions: https://en.cppreference.com/cpp/language/exceptions
+- Memory / smart pointers: https://en.cppreference.com/cpp/memory
+- Containers (iterator invalidation tables):
+  https://en.cppreference.com/cpp/container
+- Concurrency: https://en.cppreference.com/cpp/thread
+- Memory order: https://en.cppreference.com/cpp/atomic/memory_order
+- Compiler support by standard: https://en.cppreference.com/cpp/compiler_support
+- C reference (for C interop): https://en.cppreference.com/c
+
+## Other references
+
+- C++ Core Guidelines:
+  https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines
+- clang-tidy checks: https://clang.llvm.org/extra/clang-tidy/checks/list.html
+
+If a deep link 404s, use the search URL above instead.