feat(cmat): add cmake build for course material

Add a CMakeLists.txt to each exercise directory, included from
cmat/CMakeLists.txt, so every exercise and solution becomes a buildable
target. GUI exercises link the curses-based DOS_Unix implementations on
Unix-like systems; the Windows-specific DOS variants are not compiled.
Exercises without a portable path or with missing dependencies (Boost)
are reported as not available at configure time.

Targets are excluded from the default build and built by name, e.g.
`cmake --build build --target Ex11`, so intentionally buggy exercises
such as Ex21 do not break the default build.

Also fix two portability issues: rename Ex11bug/pole.h to Pole.h for
case-sensitive filesystems and drop the extra Light:: qualification in
SolutionEx71A/Light.h. Document build commands and layout in AGENTS.md.

Co-authored-by: Qwen 3.8 27B <qwen3.8_27b-q3_k_xl@lmstudio.local>

cmat/Ex11bug/Light.cpp

@@ -0,0 +1,22 @@
+
+
+#include "Light.h"
+#include "CursesObj.h"
+#include "Pelican.h"
+#include "Screen.h"
+#include <iostream>
+
+Light::Light(std::string name, int x, int y, int color) : x(x), y(y), c(color), name(name) {}
+
+Light::~Light() {
+    std::cout << "Deleting light (" << name << ", " << x << ", " << y << ", " << c << ")" << std::endl;
+    std::cout.flush();
+}
+
+void Light::set(LightState newState) {
+    state = newState;
+    Screen::write(name.c_str(), x, y, state == FLASH ? (128 + c) : (state == OFF ? 0 : c));
+    // 128 + color was used to make a character flash (but not any more)
+}
+
+Light::LightState Light::get(void) { return state; }