fix: add xattr workaround for macOS Gatekeeper warning on unsigned binaries (#2754)
Remove the com.apple.quarantine attribute after downloading cog on macOS
to prevent the 'cannot be opened because the developer cannot be verified'
Gatekeeper warning. This is a stopgap until proper code signing is added.
- tools/install.sh: clear quarantine attribute after chmod on macOS
- docs/getting-started.md: add xattr to install snippet + troubleshooting note
- docs/llms.txt: regenerated
Closes #2738