fix: harden error handling and shell-agnostic mise activation

- Guard x-path call to prevent PATH clobber when missing
- Make husky init.sh use shell-agnostic mise activate --shims
- Add fail-fast to Tier 2 install steps in dfm install all
- Check brew update exit status before reporting success
- Always run mise install during reset_nvim (handles bootstrap)
- Use double-bracket conditional in cleanup script

scripts/cleanup-old-version-managers.sh

@@ -5,7 +5,7 @@ set -euo pipefail
 # because nvim's Mason installs some of the same tool names.
 #
 # Ensure DOTFILES is set even when script is invoked directly
-if [ -z "${DOTFILES:-}" ]; then
+if [[ -z "${DOTFILES:-}" ]]; then
   DOTFILES="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
   export DOTFILES
 fi