docs: streaming client methods [AMB-3018]

[bufo24]

Summary

Documents transactions.watch(id) and wallets.watchEvents(id) (added in #46 / AMB-3016, not yet merged) across the three places this repo's "keep in sync" rule (AGENTS.md's "Docs to keep in sync") requires:

• AGENTS.md — added watch(id) / watchEvents(id) to the resource table's Transactions/Wallets rows.
• packages/payments/README.md — new "Streaming" section under API resources: minimal for await sample for each method, what a yielded PaymentEvent looks like, and the differing end-of-stream behavior (transaction stream closes on terminal status; wallet stream stays open until the 30-minute max lifetime or disconnect).
• docs/INTEGRATION.md — added streaming as "Step 6", framed as a live third option alongside webhooks and polling (per the parent Linear project's framing: webhooks need a public endpoint, SSE doesn't).

All behavioral claims (60s token TTL gating only the handshake, 30-minute max stream lifetime, terminal-status close for transactions, no-terminal-state for wallets, reconnect-by-recalling) are drawn directly from AMB-3016's implementation (streaming.ts, transactions.ts, wallets.ts doc comments) and the SSE design doc (amboss-rails-api#564), not invented.

Base branch note: this PR is based on jesseva/amb-3016-sdk-streaming-client-methods-types (#46), which is itself unmerged, so the methods being documented exist on this branch. Once #46 merges to main, this PR should be retargeted to main before merging.

Test plan

Docs-only change — no build/lint/test to run.

• Once feat: streaming client methods + types [AMB-3016] #46 merges, retarget this PR's base to main and resolve any diff drift.
• Human review of wording/accuracy against the merged streaming implementation.