Add a .gitignore — build output can be committed today #1

This repository has no .gitignore at the root, unlike Shell.Aop and Shell.Bootstrapper, which ship the standard .NET ignore file. Same gap as beyondnetcode/Shell.ddd#1.

Without it, bin/, obj/, .vs/ and *.user files can be committed by any contributor whose editor doesn't filter them.

Acceptance criteria

• .gitignore added, copied from Shell.Aop/.gitignore so the family stays consistent.
• git status is clean after running dotnet build -c Release.
• No already-tracked build output remains — verify with git ls-files | grep -E "/(bin|obj)/".

Where

Repository root.