Add a `cook completions` command for shell completion scripts

[dubadub]

Problem

cook has 14 subcommands (recipe, server, build, shopping-list, seed, search, import, report, doctor, pantry, lsp, login, logout, update), each with its own flags, but there is no shell completion. Users have to remember command names and flags or keep cook --help open.

Proposal

Add a cook completions <shell> subcommand that prints a completion script to stdout, using clap_complete — the CLI already uses clap 4.5 with the derive API, so generation is essentially free from the existing CliArgs.

cook completions bash    > /usr/local/etc/bash_completion.d/cook
cook completions zsh     > "${fpath[1]}/_cook"
cook completions fish    > ~/.config/fish/completions/cook.fish
cook completions powershell
cook completions elvish

Scope

• New src/completions.rs with CompletionsArgs { shell: clap_complete::Shell } and a run() that calls clap_complete::generate against CliArgs::command().
• New Command::Completions variant in src/args.rs + match arm in main.rs.
• Docs page under docs/ describing per-shell install (kept in sync with cooklang.org via the usual one-directional flow).

Open questions

• Feature-gated subcommands. import, server, lsp and update are behind cargo features. Generation happens off the runtime Command, so a completion script generated by a given binary will match that binary's features — worth calling out in the docs rather than working around.
• Dynamic completion of recipe names. Completing .cook files (and recipe.cook:2 scaling suffixes) for cook recipe / cook shopping-list would be the real quality-of-life win. clap_complete::engine supports dynamic completion, but it is a bigger change and could land as a follow-up. Suggest shipping the static version first.
• Shipping the scripts. Release archives (cook-*.tar.gz / .zip) currently contain just the binary. Should the build generate and bundle completion scripts so packagers (Homebrew et al.) can install them automatically?

Prior art

gh, rustup, cargo, starship all expose the same <tool> completions <shell> shape; clap_complete is the standard route for clap-based CLIs.