fix(api): defer follow-ups until Zed thread established (no divorced thread)

[lukemarsden]

Follow-up to #2660, which was incomplete. After it merged, the original reproducer (move task to backlog → re-plan from scratch → quick correction during boot) failed identically — the agent answered the correction with "a previous conversation context that I don't have."

Why #2660 missed it

#2660 guarded the interrupt poller decision site. But a second dispatch path — processAnyPendingPrompt → sendQueuedPromptToSession — is exempt from the busy-defer for interrupt prompts, and it dispatched the correction with an empty acp_thread_id concurrently with the initial (also empty-thread). Two empty-thread sends → Zed forked two threads: initial's spec work in A, correction in B. The session bound to B, which never held the initial → context loss.

Also: #2660's "validation" checked message completion + length, not whether the agent retained context, so it passed while the bug was live.

Fix — at the real chokepoint

In sendQueuedPromptToSession, the interrupt's exemption from the busy-defer is only safe once the thread exists. Gate it on threadNotEstablished := session.Metadata.ZedThreadID == "":

threadNotEstablished := session.Metadata.ZedThreadID == ""
if !prompt.Interrupt || threadNotEstablished {
    // busy-defer: don't dispatch a second empty-thread message while the
    // initial is still creating the thread; redeliver once ZedThreadID is set.
}

Until thread_created populates ZedThreadID, every prompt (interrupt included) respects the busy-defer, so only the genuine first message is ever sent empty-thread. Once the thread exists, the interrupt is exempt again and fires into the same thread.

Validation (content-verified, live Zed, meta.helix.ml)

Fresh spec task, initial = "WidgetSync … over Bluetooth", interrupt during boot = "Actually … over WiFi":

• Session bound to one thread (04101764) — no fork.
• Zero empty-thread correction sends.
• Agent's correction response: "The user wants to change the sync mechanism from Bluetooth to WiFi. Let me now write the spec files for WidgetSync that syncs widgets between devices over WiFi…" — retained the feature and applied the correction. No "context I don't have."
• Unit: TestWebSocketSyncSuite + TestPromptHistoryHandlersSuite green.

Incident report updated with this follow-up: design/2026-06-19-incident-interrupt-during-boot-context-loss.md.

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