feat(openrouter): cap max_tokens per request (OPENROUTER_MAX_TOKENS)

OpenRouter reserves credit against the requested max_tokens, falling
back to the model's full output window when omitted, so expensive
models failed with credit-limit errors even for short replies. Default
the cap to 16384; OPENROUTER_MAX_TOKENS overrides it and 0 disables it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -636,6 +636,9 @@ OPENROUTER_BASE_URL=https://openrouter.ai/api/v1     # Override base URL (option
 OPENROUTER_EMBEDDING_MODEL=openai/text-embedding-3-small  # Optional, embeddings stay local today
 OPENROUTER_HTTP_REFERER=https://your-site.example    # Optional attribution header
 OPENROUTER_APP_TITLE=ImageApi                  # Optional attribution header
+OPENROUTER_MAX_TOKENS=16384                    # Per-call max_tokens cap (default 16384; 0 = uncapped).
+                                               # OpenRouter reserves credit against this, so an uncapped
+                                               # request to a pricey model can fail on credit limits.
 
 # Local LLM backend switch. `ollama` (default) keeps the OLLAMA_* settings
 # above; `llamacpp` swaps the entire local stack (chat + vision describe +