chore: mark top-level shell scripts executable in git index

macos/build-pkg.sh, linux/install.sh, and scripts/release.sh were all
tracked with 100644 because they were originally committed from Windows
(NTFS doesn't preserve the +x bit). The release workflow invokes
./build-pkg.sh directly on macos-latest and hit EACCES. Flip the index
mode to 100755 so the bit is recorded in the tree and honored on
checkout across all platforms.

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

Author: yysjasmine