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>

src/ai/local_llm.rs

@@ -43,14 +43,16 @@ impl LocalLlm {
         )
     }
 
-    /// Embed one string via the `LLM_BACKEND`-selected client.
-    pub async fn embed(&self, text: &str) -> Result<Vec<f32>> {
-        super::embed_one(&self.ollama, self.llamacpp.as_deref(), text).await
+    /// Embed a search query (applies `EMBED_QUERY_PREFIX`). Callers must
+    /// pick query vs document — retrieval models treat the two sides
+    /// differently and an unmarked embed invites prefix-mismatch bugs.
+    pub async fn embed_query(&self, text: &str) -> Result<Vec<f32>> {
+        super::embed_query(&self.ollama, self.llamacpp.as_deref(), text).await
     }
 
-    /// Embed a batch via the `LLM_BACKEND`-selected client.
-    pub async fn embed_batch(&self, texts: &[&str]) -> Result<Vec<Vec<f32>>> {
-        super::embed_many(&self.ollama, self.llamacpp.as_deref(), texts).await
+    /// Embed corpus text (applies `EMBED_DOCUMENT_PREFIX`).
+    pub async fn embed_document(&self, text: &str) -> Result<Vec<f32>> {
+        super::embed_document(&self.ollama, self.llamacpp.as_deref(), text).await
     }
 
     /// Single-shot local text generation via the `LLM_BACKEND`-selected