enrich

Process the bookmarks `_inbox/` queue. For each file: fetch the URL via extract.py, spawn the bm:enricher subagent, parse its JSON response, and file the result into a collection. When the enricher proposes a brand-new collection, prompts the user inline (unless `--no-prompt`). Idempotent. Use when the user types /bm:enrich, says 'process the inbox', or 'enrich the bookmarks'.