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/SolutionEx71Bonus/GroupEntry.h

@@ -0,0 +1,30 @@
+// GroupEntry.h
+
+#ifndef GROUPENTRY_H
+#define GROUPENTRY_H
+
+#include <map>
+#include <string>
+#include <vector>
+
+#include "Entry.h"
+#include "PersonEntry.h"
+
+class GroupEntry : public Entry {
+  public:
+    GroupEntry(std::string name);
+
+    // print the entire group, including its members
+    // precede the name with the "indent" string
+    void print(std::string indent = "") const;
+
+    // child management
+    void addEntry(const Entry &);
+
+    void accept(EntryVisitor &) const;
+
+  private:
+    std::vector<const Entry *> _entries;
+};
+
+#endif