Fix horizon bug, multi-provider support, add Sonnet vs Gemini benchmark results

Bug fixes:
- CLI --horizon-years defaulted to 3, silently overriding config presets.
  Now defaults to None so config value (1yr for medium/hard/nightmare) is used.
- Runtime passed a single api_key kwarg regardless of provider, breaking
  Gemini. Now lets LiteLLM resolve keys from provider-specific env vars.
- Removed temperature+top_p from LLM calls (Anthropic rejects both together).
- DB and result filenames now include config name to prevent cross-config collisions.

Benchmark results (1yr horizon, 3 seeds each):
  Sonnet 4.6: medium 2/3, hard 0/3, nightmare 1/3
  Gemini Flash: medium 3/3, hard 1/3, nightmare 1/3
  Gemini has higher win rates (93-98% vs 40-83% on medium).
  Sonnet's ceiling is higher when it survives (nightmare $10.1M vs $478K).

New scripts: plot_comparison.py, plot_sonnet_results.py, notepad_gif.py
Updated README with detailed comparison tables and failure analysis.

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

src/yc_bench/agent/runtime/litellm_runtime.py

@@ -64,11 +64,12 @@ class LiteLLMRuntime(AgentRuntime):
         if self._retry_backoff_seconds <= 0:
             raise ValueError("retry_backoff_seconds must be > 0")
 
-        # API key: prefer OPENAI_API_KEY, fall back to OPENROUTER_API_KEY.
-        # For openrouter/ prefixed models LiteLLM also reads OPENROUTER_API_KEY
-        # automatically, so either approach works.
+        # API key: check provider-specific env vars, then generic fallbacks.
+        # LiteLLM reads these natively for their respective providers, but we
+        # also pass the key explicitly via kwargs to be safe.
         self._api_key = (
-            os.environ.get("OPENAI_API_KEY")
+            os.environ.get("ANTHROPIC_API_KEY")
+            or os.environ.get("OPENAI_API_KEY")
             or os.environ.get("OPENROUTER_API_KEY")
             or None
         )
@@ -159,14 +160,13 @@ class LiteLLMRuntime(AgentRuntime):
             messages=messages,
             tools=[_RUN_COMMAND_TOOL],
             tool_choice="auto",
-            temperature=self._settings.temperature,
-            top_p=self._settings.top_p,
             timeout=self._request_timeout_seconds,
         )
         if self._api_base:
             kwargs["api_base"] = self._api_base
-        if self._api_key:
-            kwargs["api_key"] = self._api_key
+        # Let LiteLLM resolve API keys from provider-specific env vars
+        # (ANTHROPIC_API_KEY, GEMINI_API_KEY, OPENROUTER_API_KEY, etc.)
+        # rather than passing a single key that may not match the provider.
 
         response = litellm.completion(**kwargs)