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>

pyproject.toml

@@ -5,6 +5,7 @@ description = "Multi-Agent System for Digital Forensics"
 requires-python = ">=3.14"
 dependencies = [
     "httpx[socks]>=0.28.1",
+    "openai>=2.36.0",
     "pyyaml",
     "regipy>=6.2.1",
 ]