Add a classic C++ Hello World example

examples/hello_world/.gitignore

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+hello_world
+hello_world.exe

examples/hello_world/Makefile

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+CXX ?= g++
+CXXFLAGS ?= -std=c++17 -Wall -Wextra -Werror -O2
+
+.PHONY: all run test clean
+
+all: hello_world
+
+hello_world: main.cpp
+	$(CXX) $(CXXFLAGS) -o $@ main.cpp
+
+run: hello_world
+	./hello_world
+
+test: run
+
+clean:
+	rm -f hello_world

examples/hello_world/README.md

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+# Hello World (C++)
+
+A classic Hello World program that prints a greeting to stdout.
+
+## Build and run
+
+Requires a C++17 compiler (`g++` or `clang++`).
+
+```bash
+cd examples/hello_world
+make run
+```
+
+Or without make:
+
+```bash
+g++ -std=c++17 -Wall -Wextra -Werror -O2 -o hello_world main.cpp
+./hello_world
+```

examples/hello_world/main.cpp

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+#include <iostream>
+
+int main() {
+    std::cout << "Hello, World!" << std::endl;
+    return 0;
+}