feat: add `.editorconfig` + `.clang-tidy` files from the `aoc25` repo

I think these files are good enough to fit most projects. other repos (github:vicinaehq/vicinae, Google's C++ programs, many-many Kotlin programs) use very similar patterns.

for someone reading this: leaving this issue as a note for myself. I have a habit of leaving public project-specific notes as issues.