Agent: add get_daily_spending for day-scoped questions

Prod showed the failure mode after the date fix: get_current_date
returned the right day, but the model answered "¿cuánto he gastado
hoy?" from cycle-level aggregates (the only spend tools it had) and
concluded ₡0. Give it a day-grained tool — per-day CRC totals mirroring
/analytics/daily-spending (COMPRA only, no installment anchors, no
future cuotas) — and a prompt rule to match tool to time grain.
Verified: fresh thread now calls get_current_date →
get_daily_spending(2026-07-04, 2026-07-05) and answers correctly.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/tests/test_agent_tools.py

@@ -183,3 +183,34 @@ def test_get_current_date_is_costa_rica_and_cycle_consistent():
     start, end = out["cycle_range"]
     assert start <= out["date"] < end
     assert start.endswith("-18") and end.endswith("-18")
+
+
+def test_daily_spending_day_scoped(bound_session):
+    from datetime import timedelta
+
+    from app.timeutil import today_cr
+
+    today = datetime.combine(today_cr(), datetime.min.time())
+    bound_session.add(
+        Transaction(amount=Decimal("100"), merchant="TODAY-A", date=today.replace(hour=9))
+    )
+    bound_session.add(
+        Transaction(amount=Decimal("50"), merchant="TODAY-B", date=today.replace(hour=15))
+    )
+    bound_session.add(
+        Transaction(
+            amount=Decimal("999"),
+            merchant="FUTURE CUOTA",
+            date=today + timedelta(days=40),
+        )
+    )
+    bound_session.commit()
+
+    out = tools.get_daily_spending(
+        start_date=today.date().isoformat(),
+        end_date=(today.date() + timedelta(days=60)).isoformat(),
+    )
+    assert len(out) == 1
+    assert out[0]["date"] == today.date().isoformat()
+    assert out[0]["total_crc"] == 150.0
+    assert out[0]["count"] == 2