Add shell completions for bash, zsh, and fish

[dviejokfs]

The CLI uses clap which supports generating shell completions out of the box via clap_complete.

Acceptance criteria

• portzero completions bash outputs bash completions
• portzero completions zsh outputs zsh completions
• portzero completions fish outputs fish completions
• Install instructions added to README or docs

References

• clap_complete docs
• CLI entry point: crates/portzero-cli/src/main.rs