List available RAG tags, categories, and filenames so you can choose real filter values before searching.

Returns facet counts for the current scope, including:
- `tags` — available tag values with document counts
- `categories` — available category values with document counts
- `filenames` — available exact filename values with document counts

Optional filters:
- `document_id` — restrict metadata discovery to a single document
- `user_id` / `agent_id` / `chat_id` — restrict discovery to this scope
- `limit` — max values returned for each field

Use this before `rag_search` when you want to filter by known metadata instead of guessing.
