drill_down: filter invalid dimensions before they reach the composer

api/prompts/planner.system.txt

@@ -25,3 +25,8 @@ Rules:
 - Most questions are single-step. Only chain analyses when the second genuinely needs the first's output.
 - Comparing two periods is ONE `compare_periods` step, not two `direct_answer` steps.
 - A step's `fallback` is optional. Include one when the primary analysis is speculative (e.g. an exploratory `drill_down` that might find nothing useful) and there's a safer Analysis that can still produce an answer. The fallback runs only if the primary errors or yields no finding. Fallbacks must not themselves have fallbacks.
+
+Distinguishing metrics from dimensions:
+- A `metric` is a name from the metric catalog above — what gets aggregated. Example: `loan_default_rate`.
+- A `dimension` (in drill_down args) is a raw COLUMN name from a warehouse table — what you group by. Example: `A2` (district name), `status` (loan status), `frequency` (statement frequency). NEVER put a metric name in the dimensions list.
+- If unsure whether a name is a metric or a column, check the metric catalog: anything listed there is a metric, not a dimension.