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

.env.example

@@ -6,21 +6,21 @@
 # ---------------------------------------------------------------
 
 # ---------------------------------------------------------------
-# AI Provider
+# AI Model (via litellm — supports any provider)
 # ---------------------------------------------------------------
-# Choose "anthropic" (Claude) or "openai" (GPT).
-AI_PROVIDER=anthropic
-
-# Optional: override the default model for the chosen provider.
-# Anthropic default: claude-opus-4-6
-# OpenAI default:    gpt-4o
-# AI_MODEL=
+# Use litellm model strings: "<provider>/<model-name>"
+# Examples:
+#   anthropic/claude-opus-4-6   (default)
+#   openai/gpt-4o
+#   gemini/gemini-2.0-flash
+AI_MODEL=anthropic/claude-opus-4-6
 
 # ---------------------------------------------------------------
-# API Keys — set the one matching your AI_PROVIDER
+# API Keys — set the one matching your chosen model's provider
 # ---------------------------------------------------------------
 ANTHROPIC_API_KEY=sk-ant-...
 # OPENAI_API_KEY=sk-...
+# GEMINI_API_KEY=...
 
 # ---------------------------------------------------------------
 # Email — IMAP (receiving parent messages)