Copies project.dev_secrets in as .env.dev and adds it to .gitignore. A blank or absent project.dev_secrets is silently skipped; a path that's set but unreadable is a real issue.

Svelixir.Config.Project.from_map/1 already resolves dev_secrets to an absolute path (or "") at config-load time, before Template cd's the process into the generated project directory -- so a relative value in config.toml still resolves against wherever mix svelixir.new was invoked from, not the generated project.