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/SolutionEx71/GroupEntry.cpp

@@ -0,0 +1,35 @@
+
+#include <iostream>
+
+#include "EntryVisitor.h"
+#include "GroupEntry.h"
+#include "PersonEntry.h"
+
+GroupEntry::GroupEntry(std::string name) : Entry(name) {}
+
+void GroupEntry::print(std::string indent) const {
+    // print group name
+    std::cout << indent << "Group: " << getName() << '\n';
+
+    // Part 1: call print() on each group member (with increased indent level)
+
+    for (auto p : _entries) {
+        p->print(indent + "  ");
+    }
+}
+
+void GroupEntry::addEntry(const Entry &entry) {
+    // Part 1: Add the entry to this group
+
+    _entries.push_back(&entry);
+}
+
+void GroupEntry::accept(EntryVisitor &v) const {
+
+    v.visit(*this);
+
+    // pass visitor to my children
+    for (auto p : _entries) {
+        p->accept(v);
+    }
+}