feat!: migrate to OpenAI native tool calls and fix circular deps (#fuck-gemini)

- Fix circular dependencies in agent/tools
- Migrate from custom JSON to OpenAI tool calls format
- Add async streaming (step_stream, complete_stream)
- Simplify prompt system and remove token counting
- Add 5 new API endpoints (/health, /v1/models, /api/memory/*)
- Add 3 new tools (get_torrent_by_index, add_torrent_by_index, set_language)
- Fix all 500 tests and add coverage config (80% threshold)
- Add comprehensive docs (README, pytest guide)

BREAKING: LLM interface changed, memory injection via get_memory()

agent/llm/exceptions.py

@@ -0,0 +1,19 @@
+"""LLM-related exceptions."""
+
+
+class LLMError(Exception):
+    """Base exception for LLM-related errors."""
+
+    pass
+
+
+class LLMConfigurationError(LLMError):
+    """Raised when LLM is not properly configured."""
+
+    pass
+
+
+class LLMAPIError(LLMError):
+    """Raised when LLM API returns an error."""
+
+    pass