Selects optimal Teams provider from chain based on real 7d utilization.
Uses cached monitor data (no extra API calls if fresh cache exists).
- Phase 1: first provider in chain with 7d util < SWITCH_THRESHOLD (default 75%)
- Phase 2: all over threshold → pick lowest 7d allowed provider
- Phase 3: all rejected → emergency=true, signals shelley-proxy needed
- Always fails safe: returns team-vigilio on any error
