feat(mobile): the Mac role, and role-dependent plugins

Looking at the actual machine turned up something the shared plugin list could
not express. The Mac loads macos, brew and dotbare; beastix and shron load
archlinux. One list filtered by availability - which is what the stub did -
would have dropped four plugins on the Mac without a word. Assembly moves into
05-pre-omz.zsh, which knows the role, and the availability filter stays on top
of it so a plugin that is not installed still cannot produce a warning at every
login.

.zprofile is added, and that is the whole point of the mobile role. The Mac
sets its PATH in .zshrc, so /opt/homebrew/bin exists for interactive shells and
for nothing else: "ssh mac brew --version" comes back empty, which during the
design phase was read as Homebrew not being installed. It is installed, with
178 formulae. Homebrew's shellenv belongs in .zprofile and now lives there; the
loop is inert on Linux, where neither prefix exists.

Carried over from the Mac: the Homebrew auto-update settings and bench(), which
runs bench inside the ERPNext compose stack.

PYTHONPATH pointed at /usr/lib/python3.9/site-packages there too - a Linux path
on macOS, so doubly wrong. All three machines had a variant of that line and
none of them is kept.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.config/zsh/rc.d/05-pre-omz.zsh

@@ -28,3 +28,25 @@ case "$ROLE" in
     zstyle ':omz:update' mode auto
     ;;
 esac
+
+# Plugins. Assembled here rather than in the stub because the set is
+# role-dependent: macos, brew and dotbare exist only on the Mac, archlinux only
+# where pacman does. A single shared list would have silently dropped four
+# plugins on the Mac.
+#
+# Filtered by what is actually present. Naming a plugin that is not installed
+# makes oh-my-zsh complain on every shell start, and the local custom plugin
+# signal-keyring is exactly that case on any machine other than beastix.
+_want=(git colored-man-pages colorize zsh-interactive-cd fzf)
+case "$ROLE" in
+  desktop|server) _want+=(archlinux) ;;
+  mobile)         _want+=(macos brew dotbare) ;;
+esac
+
+plugins=()
+for _p in $_want; do
+  if [[ -d "$ZSH/plugins/$_p" || -d "${ZSH_CUSTOM:-$ZSH/custom}/plugins/$_p" ]]; then
+    plugins+=("$_p")
+  fi
+done
+unset _p _want