build: generate fish completion at release instead of committing it

[u9g]

autocomplete/fish_autocomplete was generated from the command tree and committed. Any PR that adds or changes a command then failed CI until someone regenerated the file by hand. For example, #997 failed only because of this.

This PR stops committing the file and generates it during the release instead:

• The file is removed from the repo and added to .gitignore.
• The release job in release.yaml pulls lk out of the linux/amd64 archive and runs lk generate-fish-completion. It then adds the output to every .tar.gz and .zip before checksums are computed. The archives keep the same layout, so install-cli.sh needs no change.
  • It runs in the release job because lk only builds with cgo, and most release jobs cross-compile binaries that can't run on their runner. The fish output is the same for every target.
• The "Verify fish_autocomplete is up to date" step in test.yaml and the go:generate directive are removed.

Tested locally by running the new step on sample linux .tar.gz and windows .zip archives built with a real lk. Both came out with autocomplete/fish_autocomplete added, and it is identical to the committed file on main.

Follow-up needed: the homebrew-core livekit-cli formula installs autocomplete/fish_autocomplete from the source tarball. Before the first release without the file, the formula has to generate the completion instead:

generate_completions_from_executable(bin/"lk", "generate-fish-completion", shells: [:fish], shell_parameter_format: :none)