Build `cost_recommendation(monthly_spend, growth_rate, threshold)` returning one of `"scale"`, `"rightsize"`, or `"ok"`: 1. `growth_rate > 1.5` AND `monthly_spend < threshold` → `"scale"` (under budget AND demand is exploding — pre-buy capacity). 2. `monthly_spend > threshold * 1.2` → `"rightsize"` (>20% over budget — request/limit tuning + spot migration before scaling further). 3. `monthly_spend > threshold` AND `growth_rate < 1.0` → `"rightsize"` (over budget AND demand is shrinking — definite tuning signal). 4. Otherwise → `"ok"` (within bounds; no action this month). `growth_rate` is the month-over-month multiplier (1.2 = +20% MoM).