Add discoverPlugins() helper to @refrakt-md/cli
Replace the lazy runPlugin import-on-demand pattern in packages/cli/src/bin.ts with a first-class discoverPlugins() helper that returns the full set of installed plugins. The helper has four consumers in v0.11.0 — CLI dispatch, refrakt --help, refrakt plugins list, and the new MCP server — and centralizing the logic eliminates three different implementations of the same scan.