Add os-x native support

[mihajloS]

Summary

Add apfel as an alternative backend, enabling fully offline, zero-cost command generation using Apple's on-device LLM on macOS Tahoe.

Changes

• Add --backend apfel flag to select the local on-device model instead of the default Claude API
• Invoke apfel via child_process.execFile with a compact prompt that requests a JSON response containing the command and its safety classification
• Fall back to treating raw output as an unsafe command when the model does not return valid JSON
• Skip the Anthropic API key check when using the apfel backend
• Update README with prerequisites, shell function examples, and a backend comparison table

How to test

1. Install apfel: brew install apfel
2. Add the apfel shell function variant to ~/.zshrc:

   ask() {
     local cmd
     cmd=$(command ask-llm-cli --backend apfel "$@") || return
     print -z "$cmd"
   }