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/concurrency.md

@@ -0,0 +1,25 @@
+# Concurrency
+
+Docs: cppreference "Concurrency support library", `std::memory_order`,
+`std::atomic`.
+
+- Shared mutable data is protected by a mutex or is `std::atomic`; otherwise
+  it's a data race (UB).
+- Use `std::scoped_lock` / `std::lock_guard` / `std::unique_lock`, never manual
+  `lock()`/`unlock()`.
+- Multiple mutexes locked together with `std::scoped_lock(a, b)` or in a
+  consistent order.
+- No callbacks, I/O, or user code invoked while holding a lock.
+- Condition variables wait with a predicate (spurious wakeups).
+- `std::thread` is joined or detached before destruction; prefer `std::jthread`
+  (C++20).
+- `std::async` futures: the returned future's destructor blocks; don't discard
+  it by accident.
+- Atomics: `memory_order_relaxed` only when no other memory depends on the
+  value.
+- Lambdas passed to threads don't capture locals by reference that may die
+  first.
+- Static local init is thread-safe; other lazy init (double-checked locking)
+  needs atomics or `std::call_once`.
+- Coroutines: references/pointer parameters may dangle after the first
+  suspension.