Add Creem and expanded Stripe billing emulation

[Ansonhkg]

What changed

• Add a stateful Creem Test Mode emulator with hosted checkout, signed webhooks, subscriptions, cancellation, seed configuration, inspector UI, package documentation, and CLI registry support.
• Expand Stripe emulation for recurring prices, subscription checkout, richer checkout session responses, payment intents, invoices, subscription lifecycle routes, and versioned webhook events.
• Extend shared checkout and webhook helpers used by the billing emulators.
• Add Creem documentation to the repository README, agent skill, and docs website.
• Add focused Creem and Stripe tests for the supported billing flows.

Why

Applications need realistic local billing flows without sending requests to Stripe or Creem. The existing Stripe emulator did not model the subscription response shapes and lifecycle events required by YapHaus, and Creem was not available as an emulated service.

Impact

Developers can run subscription checkout, webhook delivery, retrieval, and cancellation locally using test state. No real payment is processed.

Validation

• pnpm --filter @emulators/creem... build
• pnpm --filter @emulators/stripe... build
• pnpm --filter @emulators/creem test with 2 tests passing
• pnpm --filter @emulators/stripe test with 23 tests passing
• pnpm --filter @emulators/creem type-check
• pnpm --filter @emulators/stripe type-check
• git diff --check upstream/main...HEAD
• Leak audit result: Safe to publish

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