TW-5118: fix AUR package build by including completions in release archive

[qasim-nylas]

Summary

• AUR package (nylas-bin) fails to build because shell completion files are missing from the release tar.gz archive
• Added completions/* to .goreleaser.yml archives.files so the generated bash/zsh/fish completions are bundled in the GitHub release archive
• The nfpms packager (deb/rpm/apk/archlinux) was unaffected because it accesses completions directly during CI build, but the AUR flow downloads the archive separately

Related docs

• https://cli.nylas.com/docs/commands/completion-bash
• https://cli.nylas.com/docs/commands/completion-zsh
• https://cli.nylas.com/docs/commands/completion-fish

Test plan

• Tag a release and verify the tar.gz archive contains completions/nylas.{bash,zsh,fish}
• Install from AUR (yay -S nylas-bin) and confirm package() succeeds
• Verify shell completions work after AUR install (nylas <tab>)

[AaronDDM]

LGTM 👍