Add native OpenRouter model support

[lewtun]

Summary

• add native openrouter/... model-id handling before the Hugging Face Router fallback
• keep openai/... reserved for direct OpenAI usage and intentionally do not wire OPENAI_BASE_URL
• update CLI model-switching help/docs and ensure OpenRouter ids bypass HF Router catalog lookup

Tests

• uv run --extra dev pytest tests/unit/test_llm_params.py tests/unit/test_cli_rendering.py
• uv run ruff check .
• uv run ruff format --check .

Refs #223, #197, #188

[github-actions]

Claude finished @lewtun's task in 3m 27s —— View job

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Review: PR #232 — Add native OpenRouter model support

No blocking issues — 3 P1. Routing change is small and well-scoped: openrouter/... short-circuits before the HF Router fallback in both _resolve_llm_params and _print_hf_routing_info, with matching tests. No P0 found in the routing/auth/agent-loop chain. The three items below are quality-of-life misses for the new prefix.

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P1 — Auth error message doesn't mention OpenRouter

agent/core/agent_loop.py:518-520 — when LiteLLM raises 401 for a missing/invalid OpenRouter key, the friendly hint enumerates only Anthropic / OpenAI / HF Router:

"  • Anthropic:   export ANTHROPIC_API_KEY=sk-...\n"
"  • OpenAI:      export OPENAI_API_KEY=sk-...\n"
"  • HF Router:   export HF_TOKEN=hf_...\n\n"

A user picking openrouter/anthropic/claude-opus-4.7 with no OPENROUTER_API_KEY set will hit this branch and get a misleading hint. Add a fourth bullet for OpenRouter so the new prefix has discoverable error guidance. Fix this →

P1 — _get_research_model doesn't step down openrouter/anthropic/...

agent/tools/research_tool.py:222-229 has explicit Sonnet step-downs for anthropic/... and bedrock/.../anthropic, but the new openrouter/anthropic/... ids fall through to "use the same model" — so a session on openrouter/anthropic/claude-opus-4.7 runs every research sub-agent on Opus too. The PR brought a new shape into a function that's pattern-matching prefixes; the step-down should be extended:

if main_model.startswith("openrouter/anthropic/"):
    return "openrouter/anthropic/claude-sonnet-4-6"

Not a correctness break (the fall-through works), but for a frontier-model-via-OpenRouter user this materially changes per-session cost. Fix this →

P1 — test_openai_base_url_is_not_forwarded doesn't test what its name (and the README) claim

tests/unit/test_llm_params.py:33-38 and README.md:64-65 together promise that openai/... is "intentionally not wired" to OPENAI_BASE_URL, so a user who has OPENAI_BASE_URL=https://openrouter.ai/api/v1 set in their env won't have OpenAI calls silently rerouted. The test only verifies that _resolve_llm_params returns {"model": "openai/gpt-5.5"} — but LiteLLM's OpenAI adapter honors OPENAI_BASE_URL from the environment regardless of what's in the params dict (no api_base override is set in llm_params.py:187-197). The invariant the README promises isn't actually enforced.

Either (a) tighten the test to assert end-to-end routing — e.g. monkeypatch litellm.acompletion and verify the resolved request URL when OPENAI_BASE_URL is set — or (b) soften the README claim to "we don't pass api_base; LiteLLM still honors OPENAI_BASE_URL from your env, set it deliberately." Right now a reader could reasonably expect protection that isn't there.

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• Branch: codex/native-openrouter-models