feat: full-stack search engine — FTS5, vector search, hybrid merge, embedding cache, chunker

The Full Stack (All Custom):
- Vector DB: embeddings stored as BLOB, cosine similarity in pure Rust
- Keyword Search: FTS5 virtual tables with BM25 scoring + auto-sync triggers
- Hybrid Merge: weighted fusion of vector + keyword results (configurable weights)
- Embeddings: provider abstraction (OpenAI, custom URL, noop fallback)
- Chunking: line-based markdown chunker with heading preservation
- Caching: embedding_cache table with LRU eviction
- Safe Reindex: rebuild FTS5 + re-embed missing vectors

New modules:
- src/memory/embeddings.rs — EmbeddingProvider trait + OpenAI + Noop + factory
- src/memory/vector.rs — cosine similarity, vec↔bytes, ScoredResult, hybrid_merge
- src/memory/chunker.rs — markdown-aware document splitting

Upgraded:
- src/memory/sqlite.rs — FTS5 schema, embedding column, hybrid recall, cache, reindex
- src/config/schema.rs — MemoryConfig expanded with embedding/search settings
- All callers updated to pass api_key for embedding provider

739 tests passing, 0 clippy warnings (Rust 1.93.1), cargo-deny clean

src/config/schema.rs

@@ -46,6 +46,49 @@ pub struct MemoryConfig {
     pub backend: String,
     /// Auto-save conversation context to memory
     pub auto_save: bool,
+    /// Embedding provider: "none" | "openai" | "custom:URL"
+    #[serde(default = "default_embedding_provider")]
+    pub embedding_provider: String,
+    /// Embedding model name (e.g. "text-embedding-3-small")
+    #[serde(default = "default_embedding_model")]
+    pub embedding_model: String,
+    /// Embedding vector dimensions
+    #[serde(default = "default_embedding_dims")]
+    pub embedding_dimensions: usize,
+    /// Weight for vector similarity in hybrid search (0.0–1.0)
+    #[serde(default = "default_vector_weight")]
+    pub vector_weight: f64,
+    /// Weight for keyword BM25 in hybrid search (0.0–1.0)
+    #[serde(default = "default_keyword_weight")]
+    pub keyword_weight: f64,
+    /// Max embedding cache entries before LRU eviction
+    #[serde(default = "default_cache_size")]
+    pub embedding_cache_size: usize,
+    /// Max tokens per chunk for document splitting
+    #[serde(default = "default_chunk_size")]
+    pub chunk_max_tokens: usize,
+}
+
+fn default_embedding_provider() -> String {
+    "none".into()
+}
+fn default_embedding_model() -> String {
+    "text-embedding-3-small".into()
+}
+fn default_embedding_dims() -> usize {
+    1536
+}
+fn default_vector_weight() -> f64 {
+    0.7
+}
+fn default_keyword_weight() -> f64 {
+    0.3
+}
+fn default_cache_size() -> usize {
+    10_000
+}
+fn default_chunk_size() -> usize {
+    512
 }
 
 impl Default for MemoryConfig {
@@ -53,6 +96,13 @@ impl Default for MemoryConfig {
         Self {
             backend: "sqlite".into(),
             auto_save: true,
+            embedding_provider: default_embedding_provider(),
+            embedding_model: default_embedding_model(),
+            embedding_dimensions: default_embedding_dims(),
+            vector_weight: default_vector_weight(),
+            keyword_weight: default_keyword_weight(),
+            embedding_cache_size: default_cache_size(),
+            chunk_max_tokens: default_chunk_size(),
         }
     }
 }