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.

commands/cpp-test.md

@@ -0,0 +1,251 @@
+---
+description: Enforce TDD workflow for C++. Write GoogleTest tests first, then implement. Verify coverage with gcov/lcov.
+---
+
+# C++ TDD Command
+
+This command enforces test-driven development methodology for C++ code using GoogleTest/GoogleMock with CMake/CTest.
+
+## What This Command Does
+
+1. **Define Interfaces**: Scaffold class/function signatures first
+2. **Write Tests**: Create comprehensive GoogleTest test cases (RED)
+3. **Run Tests**: Verify tests fail for the right reason
+4. **Implement Code**: Write minimal code to pass (GREEN)
+5. **Refactor**: Improve while keeping tests green
+6. **Check Coverage**: Ensure 80%+ coverage
+
+## When to Use
+
+Use `/cpp-test` when:
+- Implementing new C++ functions or classes
+- Adding test coverage to existing code
+- Fixing bugs (write failing test first)
+- Building critical business logic
+- Learning TDD workflow in C++
+
+## TDD Cycle
+
+```
+RED     → Write failing GoogleTest test
+GREEN   → Implement minimal code to pass
+REFACTOR → Improve code, tests stay green
+REPEAT  → Next test case
+```
+
+## Example Session
+
+````
+User: /cpp-test I need a function to validate email addresses
+
+Agent:
+# TDD Session: Email Validator
+
+## Step 1: Define Interface
+
+```cpp
+// validator/email.hpp
+#pragma once
+#include <string>
+#include <expected>
+
+enum class EmailError {
+    Empty,
+    InvalidFormat
+};
+
+std::expected<void, EmailError> validate_email(const std::string& email);
+```
+
+## Step 2: Write Tests (RED)
+
+```cpp
+// validator/email_test.cpp
+#include <gtest/gtest.h>
+#include "email.hpp"
+
+TEST(ValidateEmail, AcceptsSimpleEmail) {
+    auto result = validate_email("user@example.com");
+    EXPECT_TRUE(result.has_value());
+}
+
+TEST(ValidateEmail, AcceptsSubdomain) {
+    EXPECT_TRUE(validate_email("user@mail.example.com").has_value());
+}
+
+TEST(ValidateEmail, AcceptsPlus) {
+    EXPECT_TRUE(validate_email("user+tag@example.com").has_value());
+}
+
+TEST(ValidateEmail, RejectsEmpty) {
+    auto result = validate_email("");
+    ASSERT_FALSE(result.has_value());
+    EXPECT_EQ(result.error(), EmailError::Empty);
+}
+
+TEST(ValidateEmail, RejectsNoAtSign) {
+    EXPECT_FALSE(validate_email("userexample.com").has_value());
+}
+
+TEST(ValidateEmail, RejectsNoDomain) {
+    EXPECT_FALSE(validate_email("user@").has_value());
+}
+
+TEST(ValidateEmail, RejectsNoLocalPart) {
+    EXPECT_FALSE(validate_email("@example.com").has_value());
+}
+```
+
+## Step 3: Run Tests - Verify FAIL
+
+```bash
+$ cmake --build build && ctest --test-dir build --output-on-failure
+
+1/1 Test #1: email_validator_test .....***Failed
+    --- undefined reference to `validate_email`
+
+FAIL
+```
+
+✓ Tests fail as expected (unimplemented).
+
+## Step 4: Implement Minimal Code (GREEN)
+
+```cpp
+// validator/email.cpp
+#include "email.hpp"
+#include <regex>
+
+std::expected<void, EmailError> validate_email(const std::string& email) {
+    if (email.empty()) {
+        return std::unexpected(EmailError::Empty);
+    }
+    static const std::regex pattern(R"([a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,})");
+    if (!std::regex_match(email, pattern)) {
+        return std::unexpected(EmailError::InvalidFormat);
+    }
+    return {};
+}
+```
+
+## Step 5: Run Tests - Verify PASS
+
+```bash
+$ cmake --build build && ctest --test-dir build --output-on-failure
+
+1/1 Test #1: email_validator_test .....   Passed    0.01 sec
+
+100% tests passed.
+```
+
+✓ All tests passing!
+
+## Step 6: Check Coverage
+
+```bash
+$ cmake -DCMAKE_CXX_FLAGS="--coverage" -B build && cmake --build build
+$ ctest --test-dir build
+$ lcov --capture --directory build --output-file coverage.info
+$ lcov --list coverage.info
+
+validator/email.cpp     | 100%
+```
+
+✓ Coverage: 100%
+
+## TDD Complete!
+````
+
+## Test Patterns
+
+### Basic Tests
+```cpp
+TEST(SuiteName, TestName) {
+    EXPECT_EQ(add(2, 3), 5);
+    EXPECT_NE(result, nullptr);
+    EXPECT_TRUE(is_valid);
+    EXPECT_THROW(func(), std::invalid_argument);
+}
+```
+
+### Fixtures
+```cpp
+class DatabaseTest : public ::testing::Test {
+protected:
+    void SetUp() override { db_ = create_test_db(); }
+    void TearDown() override { db_.reset(); }
+    std::unique_ptr<Database> db_;
+};
+
+TEST_F(DatabaseTest, InsertsRecord) {
+    db_->insert("key", "value");
+    EXPECT_EQ(db_->get("key"), "value");
+}
+```
+
+### Parameterized Tests
+```cpp
+class PrimeTest : public ::testing::TestWithParam<std::pair<int, bool>> {};
+
+TEST_P(PrimeTest, ChecksPrimality) {
+    auto [input, expected] = GetParam();
+    EXPECT_EQ(is_prime(input), expected);
+}
+
+INSTANTIATE_TEST_SUITE_P(Primes, PrimeTest, ::testing::Values(
+    std::make_pair(2, true),
+    std::make_pair(4, false),
+    std::make_pair(7, true)
+));
+```
+
+## Coverage Commands
+
+```bash
+# Build with coverage
+cmake -DCMAKE_CXX_FLAGS="--coverage" -DCMAKE_EXE_LINKER_FLAGS="--coverage" -B build
+
+# Run tests
+cmake --build build && ctest --test-dir build
+
+# Generate coverage report
+lcov --capture --directory build --output-file coverage.info
+lcov --remove coverage.info '/usr/*' --output-file coverage.info
+genhtml coverage.info --output-directory coverage_html
+```
+
+## Coverage Targets
+
+| Code Type | Target |
+|-----------|--------|
+| Critical business logic | 100% |
+| Public APIs | 90%+ |
+| General code | 80%+ |
+| Generated code | Exclude |
+
+## TDD Best Practices
+
+**DO:**
+- Write test FIRST, before any implementation
+- Run tests after each change
+- Use `EXPECT_*` (continues) over `ASSERT_*` (stops) when appropriate
+- Test behavior, not implementation details
+- Include edge cases (empty, null, max values, boundary conditions)
+
+**DON'T:**
+- Write implementation before tests
+- Skip the RED phase
+- Test private methods directly (test through public API)
+- Use `sleep` in tests
+- Ignore flaky tests
+
+## Related Commands
+
+- `/cpp-build` - Fix build errors
+- `/cpp-review` - Review code after implementation
+- `/verify` - Run full verification loop
+
+## Related
+
+- Skill: `skills/cpp-testing/`
+- Skill: `skills/tdd-workflow/`