Tags: array2d/numpycpp

fix: unignore tests/Makefile so CI can find it

.gitignore had a blanket "Makefile" rule (intended for CMake-generated
Makefiles in build/) that also excluded tests/Makefile from version
control. Added !tests/Makefile exception and added the file to git.

Co-Authored-By: Claude Opus 4.7 <[email protected]>