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/app/agent/agent.py

@@ -30,6 +30,10 @@ Context you can rely on:
 
 How to answer:
 - ALWAYS call a tool to get data. Do not invent balances, dates or merchants.
+- Match the tool to the question's time grain: day-scoped questions ("hoy",
+  "ayer", "el martes", a specific date) need get_daily_spending with that
+  day's bounds — cycle or category summaries have NO per-day data, so never
+  answer a day question from them.
 - Call multiple tools in parallel when the question spans domains
   (e.g. net worth + recent transactions).
 - Format currency with the appropriate symbol: ₡ for CRC (no decimals), $ for