enhancement: evaluate larger embedding model to improve retrieval quality

[RutgerBos]

Problem

The system uses all-MiniLM-L6-v2 as its embedding model (hardcoded default in retrievers.py). This is a 6-layer, 22M parameter model optimised for speed. It performs reasonably on general semantic similarity but struggles with:

• Short, terse, factual text (typical of memory notes)
• Domain-specific terminology
• Nuanced queries that require deeper semantic understanding

Options worth evaluating

Model	Params	Notes
all-MiniLM-L6-v2 (current)	22M	Fast, low quality ceiling
all-MiniLM-L12-v2	33M	Same family, 2× layers, meaningful quality bump for low cost
all-mpnet-base-v2	109M	Best general-purpose SentenceTransformer, strong on short texts
nomic-embed-text (via Ollama)	—	Keeps everything local and on-GPU, fits the project's local-only stance

Suggested approach

1. Fix the L2 → cosine metric bug (bug: ChromaDB collection uses L2 distance instead of cosine — degrades semantic search quality #24) first so benchmarks are meaningful
2. Run a small retrieval eval against the existing memory store with each model
3. Make the model name configurable via AMEM_EMBEDDING_MODEL env var (it's already a constructor parameter — just needs wiring to the env)

Note

Switching models on an existing persistent collection requires rebuilding the index (same migration caveat as #24). The MCP server's in-memory collection rebuilds fresh each session, so it's unaffected.