feat: switch LLM client to OpenAI SDK for DeepSeek compatibility

The previous LLMClient used raw httpx + Claude Messages API (/v1/messages,
x-api-key, Anthropic SSE event types). Incompatible with DeepSeek.

Rewrite LLMClient.__init__/chat/close to use openai.AsyncOpenAI:
- /v1/chat/completions endpoint, OpenAI message format
- Bearer auth, native SDK error types
- Stream chunks via async for + chunk.choices[0].delta.content

Tool calling protocol (ReAct text-based tags) and all surrounding helpers
(_apply_progressive_decay, _fold_old_messages, _partition_tool_calls,
tool_call_loop, etc.) are unchanged — endpoint-agnostic by design.

New optional config params surfaced to config.yaml.agent:
- reasoning_effort: "high" | "medium" | "low" — DeepSeek/o1-style depth
- thinking_enabled: bool — DeepSeek extra_body.thinking switch

main.py and regenerate_report.py pass these through to LLMClient.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

main.py

@@ -219,6 +219,8 @@ async def async_main() -> None:
         model=agent_cfg["model"],
         max_tokens=agent_cfg.get("max_tokens", 4096),
         proxy=agent_cfg.get("proxy", "auto"),
+        reasoning_effort=agent_cfg.get("reasoning_effort"),
+        thinking_enabled=agent_cfg.get("thinking_enabled", False),
     )
 
     # Initialize evidence graph