feat: SDK streaming example + test [AMB-3017]

[bufo24]

Summary

• Adds packages/payments/examples/watch.ts (+ watch.cts CJS type-check counterpart), a runnable example demonstrating transactions.watch(id): mints an invoice via createReceive when TRANSACTION_ID is unset (reusing the WALLET_ID flow from receive.ts), then iterates the AsyncIterable<PaymentEvent> and logs each event as it arrives. Follows the receive.ts/send.ts pattern — runs against the live API, type-checked in CI (typecheck:examples) but not executed there, since minting a real stream token + opening a real SSE connection needs a live backend.
• Adds two offline, CI-safe regression tests to streaming.test.ts (pnpm test) covering behavior AMB-3016's own tests didn't exercise: a malformed-JSON event payload rejecting with an unwrapped NetworkError (pins the "outer catch doesn't double-wrap NetworkError" behavior), and a response body ReadableStream erroring after already yielding an event (pins the deliberate design decision, called out in AMB-3016's log, that mid-stream read-loop errors are funneled through AmbossClient.translateError the same as the pre-stream fetch). Did not duplicate AMB-3016's existing coverage (basic mint→yield, stream_closed, pre-stream 401, pre-stream abort — all already covered in streaming.test.ts/sseParser.test.ts).
• Updates examples/.env.example with TRANSACTION_ID and AGENTS.md's Examples paragraph to mention the new files. Left the AGENTS.md resource table (.watch/.watchEvents row) to AMB-3018 per that ticket's scope.
• No package.json script changes needed: typecheck:examples includes examples/*.cts via glob (auto-picks up watch.cts); test:examples only enumerates the offline webhook examples, and watch.ts is a live-API example like receive.ts/send.ts so it isn't added there.

Stacking note

This PR is stacked on jesseva/amb-3016-sdk-streaming-client-methods-types (sdk#46, still draft/unmerged) — base is that branch, not main, since this ticket's example/test build directly on the streaming client methods it adds. Once #46 merges to main, this PR will need retargeting to main.

Test plan

• pnpm install
• pnpm build
• pnpm typecheck
• pnpm typecheck:examples
• pnpm test (67/67 passing, including the 2 new cases)
• pnpm test:examples (unaffected, still passing)
• pnpm format (no diffs beyond the new test formatting)

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