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.

zshrc/fnm.zsh

@@ -1,6 +0,0 @@
-# fnm (Fast Node Manager)
-FNM_PATH="${XDG_DATA_HOME:-$HOME/.local/share}/fnm"
-[[ -d "$FNM_PATH" ]] && export PATH="$FNM_PATH:$PATH"
-if command -v fnm &> /dev/null; then
-    eval "$(fnm env --use-on-cd --shell zsh)"
-fi