feat: add receive option to Payments SDK [AMB-2753]

[bufo24]

Finding

Investigated transactions.createReceive before writing any code, since it turned up already implemented:

• The mutation (create_receive) and the SDK resource method (Transactions#createReceive) both shipped in the initial scaffolding commit, before AMB-2725 ("add sending") landed. It has been published since payments-v0.1.0.
• Verified against the backend (amboss-rails-api origin/develop, src/modules/api/payment/transaction/transaction.service.ts): executeCreateReceive calls createInvoiceForAsset(...) and mints the invoice server-side, for both sandbox and live wallets. Unlike sending, receiving needs no client-side node macaroon or team password — the backend can safely hold invoice-only node permission without gating it behind a password, whereas spending is gated behind the client-decrypted admin macaroon.
• Confirmed there's no orphaned client-side node code for receive (no addInvoice/createInvoice in packages/payments/src/node/, unlike sendLndPayment/sendAssetPayment for sending) — nothing is missing on the SDK side.
• The Webhooks resource already covers "get notified of an incoming payment" (the other half of "receive").

So createReceive is not a stub: it's a complete, correct, thin CRUD-style wrapper — the same shape as wallets.create / environments.create. It intentionally does not mirror send()'s richer SendParams/SendResult types, because send() is rich only because it orchestrates client-side macaroon decryption + node payment execution; receive has no such orchestration to hide. Adding a receive()/ReceiveParams wrapper on top would be unrequested abstraction and would fragment a published public API for no behavioral gain.

What this PR actually does

Since the capability already existed, this PR is intentionally small:

• Fixes a stale field name in the README's receive snippet (expires_in? → the real field, expires_in_seconds).
• Adds a #### Receiving section (mirroring #### Sending) with a small usage snippet, per this ticket's "a small example usage snippet is welcome."

Out of scope (left for sibling tickets under AMB-2752): a standalone runnable examples/receive.ts script ("make an example TS file") and broader API docs site content ("add API docs"), and end-to-end live verification against a running backend ("verify send+receive from scratch").

Test plan

• pnpm run build — passes
• pnpm run format:check — passes (no lint script exists in this repo)
• pnpm run typecheck — passes
• pnpm run test — all 38 tests pass (10 core + 28 payments, unchanged — no new tests added since createReceive is a trivial passthrough, same as the untested wallets.create/environments.create)

[linear]

AMB-2753 Add receive option to Payments SDK

Review in Linear