Update README for homebrew and Improve Homebrew update handling

[feiyehua]

This PR updates the Homebrew installation guidance and prevents Homebrew-managed binaries from being replaced by the built-in updater. Follow up of #207.

Changes

• Updates the README installation instructions for Homebrew.
• Detects when cc-switch is running from a Homebrew-managed prefix.
• Allows cc-switch update to still check for the latest version under Homebrew.
• Shows a package-manager-specific message when an update is available:
  brew upgrade cc-switch
• Blocks explicit self-updates like cc-switch update --version ... for Homebrew-managed installs.
• Prevents TUI update confirmation/download flows from attempting to replace a Homebrew-managed binary.
• Shows the TUI Homebrew update message as two lines for readability.
• Adds tests for CLI update behavior, TUI update handling, Homebrew-managed installs, non-Homebrew update checks, and custom repo URL update checks.

Testing

• cargo fmt --check
• cargo test --lib homebrew
• cargo test --lib check_for_update_from_repo
• cargo test --lib non_homebrew_update_check_marks_newer_version_as_regular_update
• cargo test --lib update_check_finished_for_homebrew_update_shows_brew_toast

[SaladDay]

LGTM, THANKS