fix(fnm): load managed Node from always-on zsh layer

fnm lived in interactive-only ~/.config/zshrc, so ssh/agents/zsh -c
missed fnm-managed node. Source ~/.config/zsh/*.zsh from .zshenv and
install the fragment as zsh/fnm instead.

zsh/fnm.zsh

@@ -0,0 +1,10 @@
+# fnm (Fast Node Manager) — always-on (sourced from ~/.zshenv)
+FNM_PATH="${XDG_DATA_HOME:-$HOME/.local/share}/fnm"
+[[ -d "$FNM_PATH" ]] && export PATH="$FNM_PATH:$PATH"
+if command -v fnm &> /dev/null; then
+  if [[ -o interactive ]]; then
+    eval "$(fnm env --use-on-cd --shell zsh)"
+  else
+    eval "$(fnm env --shell zsh)"
+  fi
+fi