fix: support custom zsh config paths

[wakaka6]

Summary

• Respect exported ZDOTDIR and detect XDG-style Zsh configs explicitly sourced by ~/.zshrc.
• Migrate existing CAC PATH blocks without changing unrelated shell config bytes, including legacy cleanup and uninstall handling.
• Keep installer guidance aligned and add regression coverage for Zsh, Bash, and Fish behavior.

Testing

• bash scripts/test-shell-rc.sh
• bash scripts/test-socks5h-probes.sh
• bash -n src/utils.sh src/cmd_delete.sh install.sh scripts/test-shell-rc.sh cac
• bash build.sh followed by a clean rebuild comparison
• Manual interactive Zsh verification with an isolated temporary HOME