fix(cli): generate native zsh completions

[piiq]

Summary

• replace the zsh completion output with a native _gog zsh completer
• keep the implementation small by calling gog __complete directly from zsh
• update the existing completion test to expect the native zsh marker

Why

The current gog completion zsh output is a bash compatibility shim using bashcompinit, which is awkward in zsh and does not install as a normal native zsh completion script.

Validation

• go test ./internal/cmd
• go test ./...
• go run ./cmd/gog completion zsh | sed -n '1,40p'