Finish installer portability and symlink hardening after Herdr migration #25

Follow-up extracted from stale PR #7.

Keep later: the remaining installer work in install.sh plus small config/template touches in local/env.zsh.example and related docs that make the Herdr/OpenCode/Claude setup safer and more portable across macOS/Linux. This is separate from #14 (--check/--dry-run) and should reconcile backup behavior, new symlinks, and Linux-vs-macOS branches on current main.