Add shell completions for bash, zsh, and fish

[ArLeyar]

Description

Werma uses clap for CLI argument parsing, which has built-in support for generating shell completions. Adding completion scripts would improve the developer experience significantly.

What needs to happen

1. Add a werma completions <shell> subcommand that outputs the completion script
2. Support at least: bash, zsh, fish
3. Document installation in the README

Helpful resources

• clap_complete docs
• Werma CLI definitions are in engine/src/cli.rs

Acceptance Criteria

• werma completions bash outputs valid bash completions
• werma completions zsh outputs valid zsh completions
• werma completions fish outputs valid fish completions
• README section for shell completions