chore: slim dependencies with pydantic-ai-slim[openai]

- Switch from pydantic-ai to pydantic-ai-slim[openai]
- Removes unused provider SDKs (anthropic, boto3, cohere, google, groq, huggingface)
- Production packages: 53 (down from ~158)
- Production footprint: 178MB
- Add DEPENDENCY_SLIM.md with rollback instructions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "tatlock"
-version = "1.2.0"
+version = "1.2.1"
 description = "OpenAI-compatible API with Ollama backend"
 requires-python = ">=3.12"
 dependencies = []