Public API to stop a running agent prompt or workflow run

[FredKSchott]

Summary

Add public endpoints (and matching @flue/sdk methods) to stop an in-flight agent prompt and to cancel a running workflow run.

Background & Motivation

The public route surface is observe-only today. A client can disconnect from an event stream, but the underlying agent prompt or workflow run keeps executing — and keeps spending tokens. Anyone building a UI on top of Flue (including the planned @flue/react hooks) needs an honest "stop" affordance; right now there is no way to provide one, so client APIs must omit it entirely.

Goals

• Stop an in-flight agent prompt via public HTTP (e.g. POST /agents/:name/:id/stop).
• Cancel a running workflow run via public HTTP (e.g. POST /runs/:runId/cancel).
• Emit a terminal event on the corresponding durable stream so observers learn the prompt/run was stopped.
• Expose both operations in @flue/sdk.

Example

const client = createFlueClient({ baseUrl: "https://example.com/api" });

await client.agents.stop("triage", instanceId); // interrupt the active prompt
await client.runs.cancel(runId);                // cancel a running workflow run