* fix: exclude embedding vector from MemoryRecord serialization
MemoryRecord.embedding (1536 floats for OpenAI embeddings) was included
in model_dump()/JSON serialization and repr. When recall results flow
to agents or get logged, these vectors burn tokens for zero value —
agents never need the raw embedding.
Added exclude=True and repr=False to the embedding field. The storage
layer accesses record.embedding directly (not via model_dump), so
persistence is unaffected.
* test: validate embedding excluded from serialization
Two tests:
1. MemoryRecord — model_dump, model_dump_json, and repr all exclude
embedding. Direct attribute access still works for storage layer.
2. MemoryMatch — nested record serialization also excludes embedding.