Check .gitignore covers common sensitive files

Verify that .gitignore has entries for files that should never be committed. Check for at minimum:

- .env
- node_modules/ (Node projects)
- __pycache__/ (Python projects)
- *.log

Warn if any of these are missing from .gitignore. Add internal/check/gitignore.go, register it when .gitignore exists in the project root.