Kill extract-wave truncation: 65k ceiling, hard thinking budget, reasoning-token telemetry

Job 98194fa8d215 showed every extract call hitting the 32k cap with only
~20k chars visible despite reasoning effort=low - Gemini 2.5 Pro still
burned ~25k thinking tokens per sheet.

- EXTRACT_MAX_TOKENS default 32768 -> 65536 (model output ceiling)
- new EXTRACT_REASONING_MAX_TOKENS (default 2048): OpenRouter reasoning
  max_tokens / Gemini thinking_budget; takes precedence over effort
- log per-call reasoning token counts (usage.completion_tokens_details)
  and include thinking count in the finish_reason=length marker

backend/agents/extractors.py

@@ -57,6 +57,7 @@ class SheetExtractorAgent:
             usage_tracker=self.usage,
             usage_stage="agent.extract",
             reasoning_effort=config.EXTRACT_REASONING_EFFORT or None,
+            reasoning_max_tokens=config.EXTRACT_REASONING_MAX_TOKENS or None,
         )
 
     def run(self, scope: AgentScope) -> AgentResult: