Add shell completion generation (`zenii completions`)

[sprklai]

Description

Add zenii completions <shell> command to generate shell completion scripts for bash, zsh, fish, and PowerShell using clap's built-in clap_complete crate.

Files to modify

• crates/zenii-cli/Cargo.toml — add clap_complete dependency
• crates/zenii-cli/src/main.rs — add Completions subcommand with shell argument

Acceptance criteria

• zenii completions bash outputs bash completion script
• zenii completions zsh outputs zsh completion script
• zenii completions fish outputs fish completion script
• zenii completions powershell outputs PowerShell completion script
• Output can be piped to a file: zenii completions bash > ~/.bash_completion.d/zenii

Difficulty: Medium (1-2 hours)

This project is written in Rust. See CONTRIBUTING.md for setup instructions.