Add shell completions for zsh/bash/fish

[HMAKT99]

Summary

The akf CLI doesn't have shell tab-completion. Adding completions for zsh, bash, and fish would improve developer experience.

What needs to happen

1. Generate completion scripts using Click's built-in completion support
2. Add akf --install-completion command or document manual setup
3. Test with common shells

Helpful context

• AKF CLI is built with Click (Python): python/akf/cli.py
• Click has built-in completion support: https://click.palletsprojects.com/en/8.1.x/shell-completion/
• See _AKF_COMPLETE=zsh_source akf for Click's auto-generated completions

Acceptance criteria

• akf <TAB> shows available subcommands in zsh
• akf stamp <TAB> completes file paths
• Documentation added to README or docs/