Document CLI shell auto-completion feature

[lbedner]

Summary

Every Aegis-generated project CLI has built-in shell auto-completion via Typer, but we don't document it anywhere.

The Feature

# Install completion (one-time)
my-app --install-completion

# Then get tab-completion everywhere
my-app <TAB>           # shows all commands
my-app health <TAB>    # shows subcommands  
my-app --<TAB>         # shows options

Works with bash, zsh, fish, and PowerShell.

Tasks

• Add section to CLI Reference docs explaining completion setup
• Consider mentioning in README or Quick Start
• Maybe add my-app setup completion alias command for discoverability

Why This Matters

This is a killer DX feature that comes free with Typer but users won't know about it unless we tell them.