feat: add OpenRouter LLM chat plugin (!ask, !chat)

Single-shot (!ask) and conversational (!chat) LLM commands backed by
OpenRouter's API. Per-user history (20 msg cap), 5s cooldown, reasoning
model fallback, and model switching via subcommands.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -16,4 +16,6 @@ services:
       - ./config/derp.toml:/app/config/derp.toml:ro,Z
       - ./data:/app/data:Z
       - ./secrets:/app/secrets:ro,Z
+    environment:
+      - OPENROUTER_API_KEY
     command: ["--verbose", "--cprofile"]