fix(cli): zsh completion completes on first TAB when installed to fpath

[DerDennisOP]

Summary

Follow-up to #292. After that landed, gradient <TAB> completed under bash and fish but not zsh.

Root cause: clap_complete's dynamic zsh script is built to be sourced, where it registers _clap_dynamic_completer_gradient immediately. The Nix package (installShellCompletion --zsh) instead installs it as an fpath autoload #compdef file. There, zsh registers the file itself (_comps[gradient]=_gradient) and the script only re-registers the real completer via compdef on the first TAB — so the first TAB produces nothing and only the second works. bash/fish are sourced at shell init, so they were unaffected.

Fix: gradient completion zsh now appends an autoload bridge that runs the completer on the file's first invocation, while staying inert when the script is sourced:

[[ ${funcstack[1]} = _gradient ]] && _clap_dynamic_completer_gradient "$@"

Verified end-to-end with a real zsh + zpty: the fpath-autoloaded file now completes gradient ca<TAB> -> cache on the first TAB, and the sourced form still registers _clap_dynamic_completer_gradient without firing the bridge at source time.

Test plan

• cli/tests/completion.rs::completion_zsh_bridges_autoload_first_tab asserts the appended bridge line is present.
• cargo clippy --manifest-path cli/Cargo.toml --all-targets passes clean.
• Manual: gradient completion zsh > $fpath_dir/_gradient, new shell, gradient ca<TAB> completes on first press.