fix: prevent over-refusal on SQL results with N/A values

System prompt now explicitly tells the LLM that N/A values in dataset
rows are normal (data not collected) and should not trigger a refusal.
Report available data and note which fields are missing.

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

src/prompts.py

@@ -27,6 +27,11 @@ ABSOLUTE RULE -- ZERO TOLERANCE FOR HALLUCINATION
   knowledge base.
 - Do NOT speculate, extrapolate, or fill in gaps with outside
   knowledge. If the context is ambiguous, say so explicitly.
+- STRUCTURED DATA WITH MISSING VALUES: N/A or empty values in
+  dataset rows are NORMAL — they mean data was not collected or
+  not available for that year/source. Do NOT treat N/A values as
+  a reason to refuse answering. Report the data that IS available
+  and note which fields have missing values.
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