feat: LLM router with local (Ollama) and cloud (OpenAI) backends

- lyra.config.load() reads env into a frozen Config dataclass
- lyra.llm.complete(messages, backend) routes to Ollama /api/chat or
  OpenAI chat completions
- lyra.llm.embed(texts) calls OpenAI embeddings
- .env.example switched from Anthropic to OpenAI to match available key

pyproject.toml

@@ -4,7 +4,11 @@ version = "0.1.0"
 description = "Persistent, autonomous AI assistant"
 readme = "README.md"
 requires-python = ">=3.11"
-dependencies = []
+dependencies = [
+    "httpx>=0.28.1",
+    "openai>=2.37.0",
+    "python-dotenv>=1.2.2",
+]
 
 [dependency-groups]
 dev = [