Force LF line endings via .gitattributes

Windows CI's `actions/checkout` produces CRLF files (the runner has
core.autocrlf=true) which trips Biome's format check. `* text=auto
eol=lf` pins working-tree line endings to LF everywhere, so the CI
checkout matches what Biome enforces. Also stops the "LF will be
replaced by CRLF" git warnings on the local Windows dev box.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: cinderblock

.gitattributes

@@ -0,0 +1 @@
+* text=auto eol=lf