fix(clean): keep plugin-token collection from aborting under set -e (#889)

hint_collect_claude_plugin_tokens piped `plutil -extract enabledPlugins`
and `grep` with no guard for a no-match. When ~/.claude/settings.json
exists without an enabledPlugins key, or installed_plugins.json has no
plugin tokens (the common case: Claude Code installed, no plugins), the
grep exits non-zero. Under `set -euo pipefail` that aborted the function
and, via the command substitution in show_orphan_dotdir_hint_notice, the
whole orphan-dotfile hint section of `mo clean`.

Guard both grep pipelines with `|| true` so a no-match is a non-event:
the function always returns 0 and still checks both config files. Add a
regression test covering a Claude config that has no plugins.