recon + sqlglot validator + drill_down package; guard ReAct dimension picks against candidate list

api/prompts/drill_down.next.system.txt

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+Task: pick the next dimension to slice by, in an iterative drill-down investigation.
+
+You're given the analyst's question, the candidate dimensions, the dimensions already tried (with their slice rows), and a remaining iteration budget. Decide which dimension to try next, or stop if the investigation has converged or budget is exhausted.
+
+Output: a JSON object in one fenced ```json block:
+{
+  "action": "drill" | "stop",
+  "dimension": "<dimension name from the candidate list>",   // required when action is "drill"
+  "reason": "<one sentence>"
+}
+
+Rules:
+- Pick a dimension that hasn't been tried yet and that the previous slices suggest might explain variance in the metric.
+- If every candidate has been tried, or the previous slice already shows a clear story, return action="stop".
+- Don't repeat dimensions.
+- Never invent a dimension that isn't in the candidate list.