speakers: add `sol call speakers wipe` to clear legacy encoder artifacts
Adds a one-shot destructive command for removing speaker artifacts left
over from the old encoder so the new embedder starts from a clean
journal. Without --commit the command reports what would be deleted;
with --commit it removes:
- chronicle/*/*/*/*.npz (segment embeddings)
- chronicle/*/*/*/talents/
speaker_labels.json
speaker_corrections.json
- entities/*/voiceprints.npz (per-entity voiceprints)
- entities/*/owner_centroid.npz (every principal's centroid)
- awareness/owner_candidate.npz (owner-candidate snapshot)
Follows the bootstrap/backfill idioms: --commit opts into mutation,
--json emits the structured report, and the human output lists per-
category counts plus total files/bytes. The short-help and detail-help
both carry a DESTRUCTIVE warning.
The wipe is the migration — old resemblyzer-space embeddings are not
salvageable against a new encoder, and there is no compat layer for
NPZs lacking the `encoder` provenance field.