feat(tasks): wire agent server event ingest

[tatoalo]

Problem

once the backend provides a sandbox event ingest token, the agent server needs to opt into the new sender while preserving the existing local SSE behavior and task completion signaling

Changes

• read POSTHOG_TASK_RUN_EVENT_INGEST_TOKEN from the agent server env
• construct the task run event stream sender when the ingest token is present
• enqueue broadcast and terminal task events into the sender while still serving existing SSE clients
• drain the sender during agent shutdown and task completion paths

[tatoalo]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(tasks): wire agent server event ingest #2111 👈 (View in Graphite)
• feat(tasks): add task run event stream sender #2110
• main

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[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
packages/agent/src/server/agent-server.test.ts:272-449
**Repeated `AgentServer` setup violates OnceAndOnlyOnce**

The three `signalTaskComplete` tests each repeat an identical 20+ line `new AgentServer({...}) as unknown as { eventStreamSender, posthogAPI, signalTaskComplete }` cast. Extracting a shared helper (e.g. `createSignalTaskCompleteTestServer()`) would remove the duplication and make future type-shape changes a single-line fix instead of three.

_{Reviews (1): Last reviewed commit: "feat(tasks): wire agent server event ing..." | Re-trigger Greptile}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
packages/agent/src/server/agent-server.ts:1808-1813
`TASK_COMPLETE` is included in the union type but `enqueueTaskTerminalEvent` is only ever called with `ERROR` in this PR (and nowhere else in the codebase). Listing a type option that is never exercised is a superfluous part — if it becomes needed later it can be added then.

```suggestion
  private enqueueTaskTerminalEvent(
    method: typeof POSTHOG_NOTIFICATIONS.ERROR,
    params: Record<string, unknown>,
  ): void {
```

_{Reviews (2): Last reviewed commit: "feat(tasks): wire agent server event ing..." | Re-trigger Greptile}