feat: add C++ language support and hook tests (#539)

- agents: cpp-build-resolver, cpp-reviewer
- commands: cpp-build, cpp-review, cpp-test
- rules: cpp/ (coding-style, hooks, patterns, security, testing)
- tests: 9 new hook test files with comprehensive coverage

Cherry-picked from PR #436.

rules/cpp/testing.md

@@ -0,0 +1,44 @@
+---
+paths:
+  - "**/*.cpp"
+  - "**/*.hpp"
+  - "**/*.cc"
+  - "**/*.hh"
+  - "**/*.cxx"
+  - "**/*.h"
+  - "**/CMakeLists.txt"
+---
+# C++ Testing
+
+> This file extends [common/testing.md](../common/testing.md) with C++ specific content.
+
+## Framework
+
+Use **GoogleTest** (gtest/gmock) with **CMake/CTest**.
+
+## Running Tests
+
+```bash
+cmake --build build && ctest --test-dir build --output-on-failure
+```
+
+## Coverage
+
+```bash
+cmake -DCMAKE_CXX_FLAGS="--coverage" -DCMAKE_EXE_LINKER_FLAGS="--coverage" ..
+cmake --build .
+ctest --output-on-failure
+lcov --capture --directory . --output-file coverage.info
+```
+
+## Sanitizers
+
+Always run tests with sanitizers in CI:
+
+```bash
+cmake -DCMAKE_CXX_FLAGS="-fsanitize=address,undefined" ..
+```
+
+## Reference
+
+See skill: `cpp-testing` for detailed C++ testing patterns, TDD workflow, and GoogleTest/GMock usage.