feat(embedder,reindex): centralize ST loading in kg_utils + SIGBUS fix

Root cause: dockg.build(wipe=False) generated a 1024-clause OR-delete
predicate that recursed 666 levels in LanceDB's Rust predicate evaluator,
overflowing the tokio worker-thread stack guard page on macOS (SIGBUS).

Key changes:
- DiaryKG.build() and rebuild_index() now call dockg.build(wipe=True,
  discover_similar=False) — the wipe=False recursion was the primary SIGBUS
  trigger; discover_similar=False avoids a ~4-hour O(n²) scan on 41k nodes
- _embed_shard in diary_embedder uses load_sentence_transformer() from
  kg_utils.embedder — removes 10-line manual ST load with local_files_only fix
- wrap_embedder() shares DiaryTransformer's live ST instance with DocKG,
  avoiding a second MPS allocation during build
- DiarySnapshotManager now subclasses kg_utils.snapshots.SnapshotManager
  (removes 100+ lines of duplicated dataclass/manifest code)
- diarykg reindex CLI command added: rebuilds index without re-ingesting
- save_cache() serializes embeddings row-by-row to avoid ~750 MB spike
- OMP_NUM_THREADS + TOKENIZERS_PARALLELISM set before worker pool spawn
- embed_model default: nomic-ai/nomic-embed-text-v1 → BAAI/bge-small-en-v1.5
- Version bumped 0.92.0 → 0.92.2

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
