Add shell completion support

[ahmed-sekka]

Description

Add shell completion scripts for bash, zsh, and fish.

Expected behavior

# Generate completion script
ragctl --install-completion bash
ragctl --install-completion zsh
ragctl --install-completion fish

# Usage
ragctl ch<TAB>  → ragctl chunk
ragctl chunk --str<TAB>  → ragctl chunk --strategy

Resources

• Typer has built-in completion support: https://typer.tiangolo.com/tutorial/options/completion/

Acceptance criteria

• Add --install-completion command
• Support bash, zsh, fish
• Document in README