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>

.zprofile

@@ -0,0 +1,17 @@
+# Login shells only.
+#
+# Homebrew belongs here rather than in .zshrc. .zprofile is read by login
+# shells, which includes the one behind every terminal window and, importantly,
+# is the only hook that also covers a plain "ssh host <command>" reliably
+# enough to matter. Putting brew's PATH in .zshrc leaves it missing from
+# anything non-interactive, which is how "ssh mac brew --version" came back
+# empty and was mistaken for Homebrew being absent.
+#
+# Harmless on Linux: neither prefix exists there, so the loop does nothing.
+for _brew in /opt/homebrew /usr/local; do
+  if [[ -x "$_brew/bin/brew" ]]; then
+    eval "$("$_brew/bin/brew" shellenv)"
+    break
+  fi
+done
+unset _brew