Make the embedding model swappable via env for A/B testing

Trialing Qwen3-Embedding-0.6B (1024-dim, instruct-prefixed queries)
against nomic required code changes at every hardcoded seam; now it's a
config flip plus a reembed_embeddings run.

- EMBEDDING_DIM env (default 768) replaces every hardcoded dim check:
  daily summary / calendar / search / location DAOs, Ollama batch
  validation, reembed_embeddings
- entities gains the dim guard it never had — a wrong-dim vector
  silently kills dedup/recall (cosine over mismatched lengths is 0),
  so store None and warn instead
- embed_query / embed_document split with EMBED_QUERY_PREFIX /
  EMBED_DOCUMENT_PREFIX (literal \n expanded): retrieval models treat
  the two sides differently — nomic wants search_query:/search_document:,
  Qwen3 wants Instruct:...\nQuery: on queries only. All query-side
  call sites and all corpus writers now declare their side.
- document the contract in CLAUDE.md: change the model or any of these
  vars → re-run reembed_embeddings or search is garbage

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -645,6 +645,14 @@ OPENROUTER_APP_TITLE=ImageApi                  # Optional attribution header
 # re-embedding — mixed vector spaces break similarity search.
 LLM_BACKEND=ollama
 
+# Embedding model contract. Corpus and queries must be embedded by the same
+# model with matching prefixes — after changing the embed model or any of
+# these, run `cargo run --bin reembed_embeddings` (all tables) or search is
+# garbage. Prefix values may contain a literal \n (expanded to a newline).
+EMBEDDING_DIM=768           # 768 = nomic-embed-text v1.5; 1024 = Qwen3-Embedding-0.6B
+EMBED_QUERY_PREFIX=         # nomic: "search_query: " | Qwen3: "Instruct: <task>\nQuery: "
+EMBED_DOCUMENT_PREFIX=      # nomic: "search_document: " | Qwen3: leave empty
+
 # llama.cpp / llama-swap (used when LLM_BACKEND=llamacpp). OpenAI-compatible
 # proxy hosting one or more llama-server processes. Chat models receive
 # images directly via content-parts (all models assumed vision-capable).