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/database/search_dao.rs

@@ -189,10 +189,11 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
                 .expect("Unable to get SearchHistoryDao");
 
             // Validate embedding dimensions (REQUIRED for searches)
-            if search.embedding.len() != 768 {
+            if search.embedding.len() != crate::ai::embedding_dim() {
                 return Err(anyhow::anyhow!(
-                    "Invalid embedding dimensions: {} (expected 768)",
-                    search.embedding.len()
+                    "Invalid embedding dimensions: {} (expected {})",
+                    search.embedding.len(),
+                    crate::ai::embedding_dim()
                 ));
             }
 
@@ -245,7 +246,7 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
             conn.transaction::<_, anyhow::Error, _>(|conn| {
                 for search in searches {
                     // Validate embedding (REQUIRED)
-                    if search.embedding.len() != 768 {
+                    if search.embedding.len() != crate::ai::embedding_dim() {
                         log::warn!(
                             "Skipping search with invalid embedding dimensions: {}",
                             search.embedding.len()
@@ -325,10 +326,11 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
                 .lock()
                 .expect("Unable to get SearchHistoryDao");
 
-            if query_embedding.len() != 768 {
+            if query_embedding.len() != crate::ai::embedding_dim() {
                 return Err(anyhow::anyhow!(
-                    "Invalid query embedding dimensions: {} (expected 768)",
-                    query_embedding.len()
+                    "Invalid query embedding dimensions: {} (expected {})",
+                    query_embedding.len(),
+                    crate::ai::embedding_dim()
                 ));
             }
 
@@ -406,10 +408,11 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
 
             // Step 2: If query embedding provided, rank by semantic similarity
             if let Some(query_emb) = query_embedding {
-                if query_emb.len() != 768 {
+                if query_emb.len() != crate::ai::embedding_dim() {
                     return Err(anyhow::anyhow!(
-                        "Invalid query embedding dimensions: {} (expected 768)",
-                        query_emb.len()
+                        "Invalid query embedding dimensions: {} (expected {})",
+                        query_emb.len(),
+                        crate::ai::embedding_dim()
                     ));
                 }