.gitignore and history hygiene

What gets ignored

bin/
obj/
*.user

Build outputs (bin/, obj/) are regenerable — committing them bloats every clone and breeds merge conflicts on binary files. Machine-local settings (*.user) don't describe the project. The test for what to commit: can a teammate rebuild the world from what's checked in? Source, project files, and configuration yes; artifacts no.

Messages that explain why

Fix off-by-one in SumRange

The exclusive upper bound was treated as inclusive after the
refactor; SumRange(1,5) returned 15 instead of 10.

A good message has a subject line (imperative, ~50 chars) and, when needed, a body explaining why — the diff already shows what. "fix bug" describes nothing; six months later, nobody (including you) knows which bug or why it happened.