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

config/husky/init.sh

@@ -1,2 +1,2 @@
 #!/usr/bin/env bash
-eval "$(mise activate bash --shims)"
+eval "$(mise activate --shims)"