RFC: Plan-included credit grants (Billing Phase 5)

[softmarshmallow]

Problem

Today only explicit top-ups create balance. There is no recurring/plan-included credit — so a paid plan doesn't grant AI credit, and a brand-new org starts at zero. This is both:

• the revenue path for bundled AI (a plan should include credit), and
• a first-run prerequisite — without it, a fresh org's first hosted AI call is a 402 insufficient_credits until someone manually tops up on the web.

What this RFC should settle

• Grant model — period-based (monthly) plan-included credit: its priority relative to prepaid top-ups (drain order), rollover vs. use-it-or-lose-it, and proration on plan change.
• The PLAN_GRANT tier — the priority tier referenced (as deferred) in the AI-credits master plan; the setMonthlyIncludedCredit / stopMonthlyIncludedCredit lifecycle.
• Subscription tie-in — grants are driven by subscription state through the webhook-as-source-of-truth doctrine (no server action mints credit from a subscriptions.retrieve); Stripe → Metronome ledger.
• Gate + usage — how grants interact with the entitlement gate and ingestUsageEvent.
• Reconciliation — upgrade / downgrade / cancel behavior.

References

• The AI-credits master plan (docs/wg/platform/billing/ai-credits.md) already sketches this as deferred (Phase 5 / P5) — this issue promotes it to a real design.

Related (separate issues)

• The initial-credit / trial decision (free starter credits vs. pay-first).
• The universal payment-required UX harness.

Deliverable

Extend the billing WG doc with the grant model, then implementation issues.