chore: pin text files to LF via .gitattributes

The repository already stores LF — core.autocrlf handles the CRLF worktree
on Windows checkouts — but that relies on each contributor's local config.
Make it explicit so a contributor with autocrlf disabled cannot commit CRLF
into files the installers copy verbatim onto Linux and macOS.

No content changes: `git add --renormalize .` is a no-op against the current
tree. Binary assets are excluded with `-text` so they are never filtered.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>