💸 M4 — Run on entry tiers (~€20/mo) + more providers

[leopu00]

Mission M4 from the public roadmap: the blocker is projection precision and coordinator overhead, not raw budget; every new provider also stress-tests the pacing abstraction.

Comment to claim a slice — sub-tasks:

• Kimi in-window variance analysis ([JHT-KIMI-OPTIMIZE], buffer 88→92%)
• Pay-per-use vs subscription cost validation ([JHT-COST-VALIDATION-PAYG-VS-SUB])
• "Adding a provider" guide (ADR-0002 gate)