fix: use prompt_override in gemini_client.decide() for playbook generation

decide() ignored market_data["prompt_override"], always building a generic
trade-decision prompt. This caused pre_market_planner playbook generation
to fail with JSONDecodeError on every market, falling back to defensive
playbooks. Now prompt_override takes priority over both optimization and
standard prompt building.

Closes #143

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

src/brain/gemini_client.py

@@ -410,8 +410,10 @@ class GeminiClient:
                     cached=True,
                 )
 
-        # Build optimized prompt
-        if self._enable_optimization:
+        # Build prompt (prompt_override takes priority for callers like pre_market_planner)
+        if "prompt_override" in market_data:
+            prompt = market_data["prompt_override"]
+        elif self._enable_optimization:
             prompt = self._optimizer.build_compressed_prompt(market_data)
         else:
             prompt = await self.build_prompt(market_data, news_sentiment)