Results delivery

[JSONbored]

Problem: Once a loop iteration completes, nothing packages the result for the customer.

Area: Product / API

Proposal: Deliver a completed iteration's result to the customer as a PR link, a plain-language summary, and a diff preview.

Deliverables:

• A results payload (link, summary, diff preview) produced automatically at the end of each loop iteration.

Acceptance criteria:

• A test loop's completion produces a correct, readable result payload without manual assembly.

Boundaries:

• Blocked on Task-graph → claim → code → submit wiring #4799.

---

Part of #4778.

[linear-code]

JSO-3473