v0.7.7 - Self-Update

What's new

• apfel --update - check for updates and upgrade via Homebrew with a single command
  • Detects install method (Homebrew vs source) from binary path
  • Prompts y/N before upgrading (skips prompt in non-interactive mode)
  • Falls back to manual instructions for source builds
• README: added brew upgrade apfel one-liner to install section
• 5 new integration tests for --update