Plugin manifest discovery and the plugin watcher now track folder scope and entry module path

What

• Plugin discovery from folders now records a scope: "folder" marker and resolves an entry module path when a manifest declares modulePath.
• The plugin directory watcher now tracks a per-plugin scope and entry alongside manifests, checks an admit gate before reloading a plugin, and calls watcher.retarget() after both plugin-list updates and after a reload completes.

Why

This lets Claude Code correctly track where a plugin's code actually lives and keep watching the right files and folders as plugins are added, reloaded, or changed, instead of relying only on a static manifest list.