[P5] savings() tool — savings rate + monthly trend

Summary

Add a new savings_summary MCP tool that returns the user's savings rate and a monthly savings trend.

Requirements

1. Add a new MCP tool savings_summary(period: str) in server/main.py:

   • Accepts same period strings as summary(): month, year, ytd, YYYY-MM, YYYY
   • Returns:

     {
       "period": "2026-05",
       "income": 5000.00,
       "savings": 800.00,
       "savings_rate": 16.0,
       "monthly_trend": [
         {"month": "2026-03", "income": 5000, "savings": 700, "rate": 14.0},
         {"month": "2026-04", "income": 5000, "savings": 750, "rate": 15.0},
         {"month": "2026-05", "income": 5000, "savings": 800, "rate": 16.0}
       ]
     }
     

   • savings = sum of transaction_entries.amount where entry_type = 'savings'
   • savings_rate = (savings / income) × 100, rounded to 1 decimal; 0.0 if income is zero
   • monthly_trend = last 3 full months (always 3 entries, 0-filled if no data)

2. Register the tool with @mcp.tool decorator

3. Add to SKILL.md under the appropriate section

4. Add tests in tests/ covering the calculation and edge cases (zero income, no savings data)

5. Update ARCHITECTURE.md MCP Tools section

Interactive check

Call the tool directly in Python and confirm it returns real output (not not_implemented).

Depends on

#281 (savings item_type must exist before this tool is meaningful)

Start in server/main.py by examining the existing summary() period handling and MCP tool registration. Add tests under tests/ for savings-rate calculation, zero income, missing savings data, and three zero-filled monthly entries. Update SKILL.md and ARCHITECTURE.md as requested, then call the tool directly in Python to confirm it returns the specified real output.