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>

AGENTS.md

@@ -10,8 +10,22 @@ is added, document its layout under "Source Layout".
 
 ## Build System
 
-No build system is set up yet. When one is added for the course examples,
-document its configure, build, and test commands here.
+CMake (3.21+) with Ninja.
+
+```sh
+cmake -B build -G Ninja
+cmake --build build
+```
+
+Course exercises in `cmat/` are excluded from the default build; build one by
+target name, e.g.:
+
+```sh
+cmake --build build --target Ex11
+```
+
+Tests (optional): configure with `-DBUILD_TESTING=ON`, then run `ninja check` in
+the build directory.
 
 ## Coding Conventions
 
@@ -33,23 +47,23 @@ document its configure, build, and test commands here.
 - **Naming:** `SCREAMING_SNAKE_CASE` only for macros and constants
 - **Header extensions:** C++ headers use `.hpp`, C headers use `.h`
 - Include order:
-    1. C++ standard library headers (`<chrono>`, `<vector>`, etc.)
-    2. _(blank line)_
-    3. C standard library headers (`<stdlib.h>`, `<string.h>`, etc.)
-    4. _(blank line)_
-    5. OS-specific headers (Windows API, POSIX, etc.)
-    6. _(blank line)_
-    7. Third-party dependencies (`"fmt/core.h"`, etc.)
-    8. _(blank line)_
-    9. Local/project headers
+  1. C++ standard library headers (`<chrono>`, `<vector>`, etc.)
+  2. _(blank line)_
+  3. C standard library headers (`<stdlib.h>`, `<string.h>`, etc.)
+  4. _(blank line)_
+  5. OS-specific headers (Windows API, POSIX, etc.)
+  6. _(blank line)_
+  7. Third-party dependencies (`"fmt/core.h"`, etc.)
+  8. _(blank line)_
+  9. Local/project headers
 
 ## Shell Scripts
 
 - Always use `#!/bin/sh` shebang for shell scripts
 - Scripts must be POSIX compliant (no bashisms)
 - When providing commands to users:
-    - Windows/PowerShell: use `` ` `` for line continuation
-    - Unix/Linux/macOS: use `\` for line continuation
+  - Windows/PowerShell: use `` ` `` for line continuation
+  - Unix/Linux/macOS: use `\` for line continuation
 
 ## Commit Messages
 
@@ -84,7 +98,7 @@ releases of the course material.
 
 - All markdown files are formatted with **oxfmt**
 - **Always run `npx oxfmt --write .` after editing any markdown file**
-    - This is mandatory. Do not skip it. Run it before finishing your response
+  - This is mandatory. Do not skip it. Run it before finishing your response
 - Use `npx oxfmt --check .` to verify formatting before committing
 - Config is in `.oxfmtrc.json` at the project root
 - Do not manually fight the formatter. Let oxfmt handle spacing, line breaks,
@@ -98,10 +112,25 @@ cppcourse/
   README.md         # Project docs
   .oxfmtrc.json     # oxfmt config
   .agents/          # Agent skills (git workflow)
+  CMakeLists.txt    # Build system entry point
+  deps/             # CMake helper modules (platform, flags, Find* deps)
+  cmat/             # Course material: exercises and solutions
+  397/              # Original copy of the course material (reference only)
 ```
 
-Course content (lectures, exercises, examples) will be added as the course is
-developed. Document the layout here when it takes shape.
+`cmat/` layout:
+
+- `Ex*/`: exercise starting points. Some intentionally contain bugs that
+  students fix during the course; their targets fail to compile (or misbehave at
+  runtime) until fixed.
+- `SolutionEx*/`: working solutions of the exercises.
+- `DOS_Unix/`: platform Keyboard/Screen implementations shared by the GUI
+  exercises. The DOS variants are Windows-specific; on Unix-like systems the
+  curses variants are built instead (linked as the `dos_unix` library).
+- Each exercise directory holds a `CMakeLists.txt` that defines its target via
+  `cmat_add_exercise()` (see `cmat/CMakeLists.txt`). Exercises that need a
+  missing dependency (e.g. Boost) are reported as not available at configure
+  time and skipped.
 
 ## Behavioral Guidelines