feat(zsh): recover Arch site-functions + HELPDIR after removing system zsh

Switching to nix's zsh on the Arch host left two functional gaps the
Arch zsh package used to fill:

1. /usr/share/zsh/site-functions in fpath: pacman, paru, systemctl,
   journalctl, flatpak, docker, kubectl, makepkg etc. drop their
   completions there. nix zsh's compiled-in fpath doesn't include
   /usr/share so we lose all of them silently. Added that path (and
   vendor-completions for the VM's apt-installed completions) to the
   existing fpath loop, guarded by [[ -d ]].

2. HELPDIR for the run-help / help-alias machinery: needed so 'help cd'
   etc. find the per-builtin help docs. Pick the first existing version
   dir, preferring nix-profile so it matches the running zsh version.

1 files changed, 18 insertions, 3 deletions

diff --git a/dot_config/zsh/dot_zshrc b/dot_config/zsh/dot_zshrc
index 961a5b4..b1021eb 100644
--- a/dot_config/zsh/dot_zshrc
+++ b/dot_config/zsh/dot_zshrc
@@ -42,13 +42,28 @@ PROMPT='%B%{$fg[green]%}%n%{$reset_color%}@%{$fg[cyan]%}%m%{$reset_color%}:%b%{$
 
 # ── Completion ────────────────────────────────────────────────────────────────
 fpath=($XDG_DATA_HOME/zsh/completion $fpath)
-# Pick up completions shipped by nix-installed packages on the remote-dev VM
-# (Ubuntu's system zsh doesn't add the nix-profile share dirs to fpath).
+# Pull in completions from both nix-installed packages AND the system
+# package manager (pacman on Arch host, apt on Ubuntu VM). nix zsh's
+# default fpath only includes its own nix-store dirs, so without these
+# entries we miss completions for pacman, paru, systemctl, flatpak,
+# docker, kubectl, etc. on the host, and apt/snap on the VM.
 for _d in "$HOME/.nix-profile/share/zsh/site-functions" \
-          "$HOME/.nix-profile/share/zsh/vendor-completions"; do
+	"$HOME/.nix-profile/share/zsh/vendor-completions" \
+	/usr/share/zsh/site-functions \
+	/usr/share/zsh/vendor-completions; do
 	[[ -d $_d ]] && fpath=($_d $fpath)
 done
 unset _d
+# HELPDIR: location of per-builtin help docs used by run-help/`help`.
+# Pick whichever exists, preferring nix-profile (newer zsh version
+# matches our running shell). The glob expands to the version-specific
+# dir, e.g. ~/.nix-profile/share/zsh/5.9/help.
+# shellcheck disable=SC1036,SC1058,SC1072,SC1073  # zsh glob qualifier (N)
+for _d in "$HOME"/.nix-profile/share/zsh/*/help(N) \
+	/usr/share/zsh/*/help(N); do
+	[[ -d $_d ]] && { export HELPDIR=$_d; break; }
+done
+unset _d
 autoload -Uz compinit && compinit -d "$XDG_CACHE_HOME/zsh/zcompdump"
 
 zstyle ':completion:*' menu select                       # arrow-key driven menu for ambiguous completions