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/report-template.md

@@ -0,0 +1,23 @@
+# Report Template
+
+```
+## Summary
+One or two sentences: overall verdict and biggest risk.
+
+## Findings
+### [Critical|High|Medium|Low] Short title: path/file.cpp:LINE
+What's wrong and why it matters (1-3 sentences).
+Fix:
+    // suggested code
+Ref: <cppreference link if helpful>
+
+## Questions
+- Things that need the author's intent to judge.
+
+## Nits (optional)
+- Minor idiom/readability notes.
+```
+
+Severity guide: **Critical** = UB, memory corruption, security; **High** = data
+race, deadlock, leak, crash; **Medium** = error handling, API, perf; **Low** =
+idioms.