fix: respect CARGO_HOME in install script

[xfocus3]

Summary

• Use ${CARGO_HOME:-$HOME/.cargo}/bin as the install script's default destination.
• Keep --to as an explicit override.
• Update the help text to describe the CARGO_HOME-aware default.

Closes #279.

Verification

• sh -n ci/install.sh
• Fake curl/tar harness verified:
  • CARGO_HOME=/tmp/custom defaults to /tmp/custom/bin
  • unset CARGO_HOME defaults to $HOME/.cargo/bin
  • --to /tmp/explicit still overrides both
• git diff --check