fix: purge stale drawers before re-mine to avoid hnswlib segfault (#521)

Delete existing drawers for a file before re-inserting fresh chunks.
Converts re-mines from upsert (hnswlib updatePoint path, thread-unsafe
on macOS ARM + chromadb 0.6.3) into delete+insert (safe addPoint path).

Credit: @StefanKremen (#523)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>