feat: SQLite-backed memory with brute-force cosine recall

- lyra.memory.remember(session_id, role, content) embeds and stores
- lyra.memory.recent(session_id, n) returns the last N from a session
- lyra.memory.recall(query, k, session_id=None) returns top-k by cosine
  similarity across the chosen scope (all sessions by default)
- Embeddings live in the exchanges.embedding BLOB column as float32 bytes
- Connection reopens automatically if LYRA_DB_PATH changes (test-friendly)

pyproject.toml

@@ -6,6 +6,7 @@ readme = "README.md"
 requires-python = ">=3.11"
 dependencies = [
     "httpx>=0.28.1",
+    "numpy>=2.4.5",
     "openai>=2.37.0",
     "python-dotenv>=1.2.2",
 ]