Return the OS-appropriate cache directory for Deep Agents Code.
Uses ~/Library/Caches on macOS, LOCALAPPDATA on Windows (falling back
to ~/AppData/Local), and XDG_CACHE_HOME elsewhere when it is an
absolute path (falling back to ~/.cache). The XDG spec treats relative
XDG_CACHE_HOME values as invalid, so they are ignored rather than
resolved against the launch directory.
Platform-native locations are the convention for a long-lived app (this is
what platformdirs codifies and what uv itself does — its own cache is
~/Library/Caches/uv on macOS). The install script deliberately does not
follow this: as a portable one-shot POSIX bootstrap it uses XDG-style
${XDG_CACHE_HOME:-~/.cache} on every platform (like the rustup and uv
installers), so on macOS its <cache>/deepagents-code/install.log lands
under a different root than the update logs. That divergence is
intentional; do not "fix" one side to match the other without a concrete
need (e.g., a diagnostic command that collects both logs).
