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/SolutionEx71ABonus/PowerVisitor.cpp

@@ -0,0 +1,23 @@
+#include "PowerVisitor.h"
+#include "Light.h"
+#include "Pole.h"
+#include <iostream>
+
+void PowerVisitor::visit(Light &l) {
+    switch (l.get()) {
+    case Light::ON:
+        runningTotal += 60;
+        break;
+    case Light::FLASH:
+        runningTotal += 30;
+        break;
+    }
+}
+
+void PowerVisitor::calculatePower(Pole &p) {
+    runningTotal = 0;
+
+    p.accept(*this);
+
+    std::cout << "Power usage is " << runningTotal << " watts.\n";
+}