fix: tighten receipt amount extraction prompt to reduce OCR misreads

Replaced 'pick the largest one' guidance with 'bottom-most total' and
'return 0 if no clear total found' to avoid picking line items or tips.

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

agent_service/agents/expenses_agent.py

@@ -327,9 +327,10 @@ class ExpensesAgent(BaseAgent):
                 'Return ONLY valid JSON with these keys:\n'
                 '"vendor" (string, merchant or restaurant name),\n'
                 '"amount" (number — the FINAL total the customer paid; '
-                'this is labeled "Total", "Amount Due", "Grand Total", or the last dollar figure; '
-                'do NOT use subtotal, tax, or tip separately; '
-                'if multiple totals appear pick the largest one labeled as the final total),\n'
+                'look for a line explicitly labeled "Total", "Grand Total", "Amount Due", or "Balance Due"; '
+                'do NOT use subtotal, tax, tip, or individual line items; '
+                'if the label is ambiguous choose the bottom-most total on the receipt; '
+                'return 0 if no clear total is found),\n'
                 f'"date" (string YYYY-MM-DD, use {date_hint or today} if not found in text),\n'
                 '"time" (string HH:MM in 24-hour format — the transaction time printed on the receipt; '
                 'null if not present),\n'