Let users add trusted plugin source repos and add/update/remove plugins
from them, alongside the existing in-shell plugin commands:
omarchy plugin source <add|list|remove|refresh>
omarchy plugin available
omarchy plugin add | update | remove | validate
Each command is interactive (gum/fzf pickers, confirmation, an update
diff) in a TTY and fully flag-driven with --yes for scripts and agents.
Sources live in ~/.config/omarchy/plugins/sources.json and clone into
~/.cache/omarchy/plugin-sources/. The installer only copies files,
validates manifests against the shell's schema, and toggles enabled
state over IPC -- it never runs plugin code, hooks, or sudo.
Also guard 'plugin bar add' so only known bar-widget ids enter the
layout (rejecting typos/non-widgets like a stray '--help').
