Add shell completion support

[huseyinbabal]

Summary

• Add shell completion support for bash, zsh, fish, and PowerShell
• New taws completion <shell> subcommand to generate completion scripts

Usage

# Bash (add to ~/.bashrc)
eval "$(taws completion bash)"

# Zsh (add to ~/.zshrc)
eval "$(taws completion zsh)"

# Fish (add to ~/.config/fish/config.fish)
taws completion fish | source

# PowerShell (add to $PROFILE)
taws completion powershell | Out-String | Invoke-Expression

Features

• Tab completion for all CLI flags (--profile, --region, --log-level, etc.)
• Tab completion for flag values (e.g., log levels: off, error, warn, info, debug, trace)
• Subcommand completion (completion, help)

Example

$ taws --pr<TAB>
$ taws --profile 

$ taws --log-level <TAB>
off    error  warn   info   debug  trace

$ taws comp<TAB>
$ taws completion 

$ taws completion <TAB>
bash  zsh  fish  powershell

Closes #97