Complete build wiring and CLI file commands

Align documented commands with the CLI, enable optional GUI/test targets, and remove committed API secrets so the project is safer to build and run.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

dstalk-gui/CMakeLists.txt

@@ -13,3 +13,9 @@ target_link_libraries(dstalk-gui
         dstalk
         SDL3::SDL3
 )
+
+add_custom_command(TARGET dstalk-gui POST_BUILD
+    COMMAND ${CMAKE_COMMAND} -E copy_if_different
+        $<TARGET_FILE:dstalk>
+        $<TARGET_FILE_DIR:dstalk-gui>
+)