release: 0.7.6 (fix permission denied on global install)

The 0.7.5 tarball shipped `dist/bin/codegraph.js` without the executable
bit set, causing `zsh: permission denied: codegraph` after a fresh global
install. The build script now `chmod +x`'s the binary before packing.

Also adds CHANGELOG.md and documents the release workflow in CLAUDE.md.