v0.4 follow-up: replace token-cosine with real bge-m3 embedding for PPA retrieval

[MakiDevelop]

Background

v0.4 PPA `_stage2_retrieve()` uses a placeholder cosine over token-count vectors. The persona_triples schema already has an `embedding BLOB` column reserved.

What's needed

• Lazily compute bge-m3 1024-dim embeddings for new triples on insert
• Store as BLOB in the existing column
• Update `_stage2_retrieve()` to use the BLOB embeddings + numpy cosine

Why this matters

Token-count cosine is good enough to demo PPA but won't match the paper's reported C-Score gains. Real embeddings should close most of that gap.

Acceptance criteria

• Embedding generation utility (local model preferred; HuggingFace inference API as fallback)
• Migration script to backfill embeddings for existing rows
• `_stage2_retrieve()` reads BLOB, uses numpy cosine
• Threshold (default 0.2) re-calibrated for bge-m3 cosine distribution

Related

This becomes moot if #5 (Mem0 integration) lands first — Mem0 handles embedding + retrieval. If we go straight to Mem0, close this issue.

Estimated effort

~1 day if we keep the SQLite path; ~0 if we jump to Mem0.