Cost center spend trends

# Spend trends by cost center

## Purpose
Analyze the month-over-month evolution of spend for each cost center. Identify cost centers whose spend is growing abnormally and those underspending their budget.

## Instructions

### Step 1 — Retrieve the list of cost centers
```
Call get_cost_centers.
For each cost center: note id, name, code.
Exclude archived centers if the API allows it.
```

### Step 2 — Analyze spend for the last 6 months
```
For a month-over-month analysis over 6 months:
  Call spendesk_analyze_spend with:
    - groupBy: costCenter
    - period: last 6 complete months (M-6 to M-1)
    - granularity: monthly (if available) or make 6 monthly calls

  For each month M:
    Spend by cost center: [centreId, month, amount, currency]
```

### Step 3 — Calculate the trends
```
For each cost center with at least 3 months of data:
  Growth M vs M-1 = (spend_M - spend_M-1) / spend_M-1 × 100
  6-month trend = simple linear regression (or growth M vs M-6)
  Volatility = monthly standard deviation / monthly average (coefficient of variation)

Trend categories:
  📈 Strong increase: growth M vs M-1 > +20%
  ↗️ Moderate increase: growth between +5% and +20%
  ➡️ Stable: variation between -5% and +5%
  ↘️ Moderate decrease: decrease between -5% and -20%
  📉 Strong decrease: decrease > -20%
```

### Step 4 — Identify anomalies
```
Anomalies to flag:
  🔴 Sudden spike: current month > 3-month average × 1.5
  🟡 Sustained upward trend: 3 consecutive months of growth > +10%
  💡 Notable underspend: current month < 3-month average × 0.5
  ⚠️ Reactivated inactive center: no spend for 2+ months, then activity
```

### Step 5 — Trend report
```
Present:
1. Summary table:
   Cost center | M-5 | M-4 | M-3 | M-2 | M-1 | M | Trend | M/M variation

2. Top 5 fastest-growing centers (with % and absolute amount)
3. Top 5 centers with a significant decrease
4. Detected anomalies with explanation and recommended action

5. Overview:
   Total spend M vs M-1: X€ (±Y%)
   Centers increasing: X | Decreasing: X | Stable: X
```

## Points to watch
- Cost centers with no spend for a month don't always appear in the API — treat them as 0€
- Compare trends to annual budgets when available for context
- Some increases are normal (year-end spike, seasonality)

## Example prompts
- "Which cost centers have the biggest spend increases this quarter?"
- "Show me the spend evolution of each BU over the last 6 months"
- "Are there any cost centers spiraling out of control?"