Follow-up 1 from #592 (the other follow-up there, pluginDataDir, stays tracked in #592's thread).

Motivation

Since #593 / 0.0.216, jss start -c config.json mounts plugins — but the simple case still requires writing a config file for what is conceptually one argument. A repeatable flag closes that:

jss start --root ./data --public --plugin ./chat/plugin.js@/chat --plugin tideholm/jss-plugin/tideholm-jss.js@/tideholm

It's also the seam servejss needs to stay a pure flag translator: with this flag, servejss --plugin ./chat/plugin.js@/chat ./public forwards verbatim, and jsserve never learns plugin semantics (consistent with its How-it-works contract: JSS owns loading, the wrapper owns the CLI).

Proposed shape

• Repeatable: commander collect pattern; each occurrence appends one entry.
• Syntax: module[@prefix] — split on the last @ whose remainder starts with /, so scoped packages parse unambiguously (@scope/pkg/plugin.js@/app → module @scope/pkg/plugin.js, prefix /app; bare @scope/pkg/plugin.js → no prefix).
• Merging: CLI entries append to config.plugins from the file (both sources usable together), rather than the usual CLI-replaces-file scalar rule — replacing would make -c plus one --plugin silently drop the file's apps.
• Scope limits: per-plugin config objects and explicit id stay config-file territory; the flag covers the module + prefix case only. Entries go through the existing loader untouched, so prefix validation, duplicate-id rejection, and fail-loudly-on-import-error all apply as-is.

Non-goals

• No change to loader semantics (src/plugins.js).
• No config auto-discovery — same explicit-opt-in stance as -c (CLI silently drops plugins and appPaths from config — #206 loader unreachable from jss start #592).