Replace OpenRouter (openai SDK) with OpusAgent REST API (httpx + polling).
LLMClient now creates topics per system prompt, submits async requests,
and polls for completion. Model tiers map to Claude model IDs
(opus-4-7, sonnet-4-6, haiku-4-5) configurable via env vars.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>