docs: add shell completion instructions to README

[mendsec]

Context

Cobra generates shell completions via catnet completion bash|zsh|fish|powershell. This is not documented anywhere in the README. Add a Completions section with copy-paste install instructions for bash, zsh, and fish.

Acceptance criteria

• README contains ## Shell Completions section
• Instructions tested on bash 5.x, zsh 5.9, fish 3.x
• catnet completion --help output matches documented usage