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/false-positives.md

@@ -0,0 +1,20 @@
+# Avoiding False Positives
+
+Before reporting, check:
+
+- A raw pointer that is non-owning (an observer) is fine; only owning raw
+  pointers are a problem.
+- Unsigned integer overflow is defined (wraps); only signed overflow is UB.
+- `std::move` on a `const` object or a return of a local is a
+  pessimization/no-op, not a bug.
+- Copy elision: returning a local by value is fine; don't suggest `std::move` on
+  return.
+- Static locals are initialized thread-safely since C++11.
+- Code guarded by `#ifdef` or feature macros may target a different standard or
+  platform.
+- If the compiler (with warnings enabled) would reject or warn on it, defer to
+  the compiler.
+- If you're unsure a construct is UB, look it up on cppreference before
+  reporting.
+
+If still unsure, mark the finding as **Question** instead of **Issue**.