build: pin shell scripts to LF via .gitattributes

Ensures install.sh keeps LF line endings so its shebang doesn't break
when served to linux/macOS users, regardless of a contributor's autocrlf
setting.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: intincrab

.gitattributes

@@ -0,0 +1,2 @@
+# shell scripts must stay LF or their shebang breaks on linux/macOS
+*.sh text eol=lf