Replace custom provider abstraction with litellm

Drops the src/providers/ package (base class, AnthropicProvider,
OpenAIProvider, factory) in favour of a single src/llm.py that calls
litellm.completion() directly. litellm handles provider routing,
authentication, and SDK differences for 100+ providers without any
code we need to maintain.

Changes:
- Delete src/providers/ entirely
- Add src/llm.py — one complete() function wrapping litellm
- src/agent/core.py: EmailAgent takes model: str instead of LLMProvider
- src/config.py: ai_provider + api key fields → single ai_model string
  in litellm format (e.g. "anthropic/claude-opus-4-6")
- main.py: remove provider factory wiring; pass config.ai_model to agent
- .env.example: simplify AI section, show litellm model string examples
- pyproject.toml: replace anthropic + openai deps with litellm>=1.0.0
- uv.lock: regenerated

https://claude.ai/code/session_01HaUFs7SaLD5SoiuGCY27Tw

src/llm.py

@@ -0,0 +1,22 @@
+"""LLM completion via litellm — supports any provider with a single call."""
+
+import litellm
+
+
+def complete(model: str, system: str, messages: list) -> str:
+    """Call any LLM and return the response text.
+
+    Args:
+        model: litellm model string, e.g. "anthropic/claude-opus-4-6" or
+               "openai/gpt-4o". The matching API key must be set as an
+               environment variable (ANTHROPIC_API_KEY, OPENAI_API_KEY, …).
+        system: System prompt text.
+        messages: List of objects with .role and .content attributes.
+
+    Returns:
+        The model's reply as a plain string.
+    """
+    api_messages = [{"role": "system", "content": system}]
+    api_messages += [{"role": m.role, "content": m.content} for m in messages]
+    response = litellm.completion(model=model, messages=api_messages, max_tokens=2048)
+    return response.choices[0].message.content