Agent: fix date handling — Costa Rica timezone and future cuotas

Two bugs from prod (server clock is UTC, user is UTC-6):

- get_recent_transactions returned future-dated Tasa Cero cuotas; it now
  excludes anything dated after the user's today, same rule as
  /transactions/recent.
- "¿Cuánto he gastado hoy?" answered for the wrong day: the prompt baked
  date.today() in at boot — frozen from startup AND in server-UTC (8 pm
  in Costa Rica is already tomorrow in UTC). The prompt no longer carries
  a date; a new get_current_date tool returns today in CR time plus the
  active billing cycle, and the prompt directs the model to call it for
  any relative date reference. today_cr() lives in timeutil (CR has no
  DST, fixed UTC-6).

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

backend/app/agent/agent.py

@@ -2,8 +2,6 @@
 
 from __future__ import annotations
 
-from datetime import date
-
 from agent_framework import Agent
 from agent_framework.openai import OpenAIChatCompletionClient
 
@@ -22,8 +20,10 @@ Context you can rely on:
   following month. When the user says "this month" or "last month" without
   qualifiers, assume they mean the calendar month unless they mention
   "cycle", "corte", or their credit card.
-- Today's date is {today}. Use it when the user says "this month", "last
-  month", "last year", etc.
+- You do NOT have a reliable clock, and the server runs in UTC while the
+  user lives in Costa Rica (UTC-6). Whenever the question involves a
+  relative date — "hoy", "ayer", "este mes", "este ciclo", "el año
+  pasado" — call get_current_date FIRST and derive ranges from its answer.
 - Amounts are stored as raw numbers in their native currency (see `currency`
   field on transactions/accounts). Tools that return `total_crc` are already
   converted; tools that return per-transaction amounts are NOT.
@@ -69,7 +69,10 @@ def build_agent() -> Agent:
     )
     return Agent(
         name="wealthysmart",
-        instructions=SYSTEM_PROMPT.replace("{today}", date.today().isoformat()),
+        # No date baked in: build_agent() runs once at startup, so anything
+        # substituted here freezes at boot (and in server-UTC). The model
+        # gets "today" from the get_current_date tool instead.
+        instructions=SYSTEM_PROMPT,
         client=client,
         tools=TOOLS,
     )