Support one-time automation triggers #605

Summary

Retrospective tracking issue for the one-time automation trigger protocol support delivered by #603, which has already merged.

This upstreams the protocol support needed for Send Later and agent-authored scheduling, rather than leaving it as a private extension in VS Code.

Delivered scope

• Add AutomationTriggerKind.OneTime and AutomationOneTimeTrigger with id, kind: oneTime, runAt, and optional misfirePolicy.
• Advertise host support through the AutomationCapabilities.oneTime presence capability.
• Define at-most-once execution for each runAt; changing runAt moves the occurrence and cancels the previous scheduled time.
• Reuse the existing missed-execution policy, defaulting to runOnce, and record the intended time as origin.scheduledFor.
• Update protocol documentation, schemas, generated clients, and shared round-trip fixtures.

Out of scope

Existing-chat/session targets, dispatch receipts, conditional updates, scheduled-chat purpose fields, and client/reconnect capability negotiation are not included. Existing-session and chat targeting remain tracked separately in #572.

Resolution

Implemented by #603. This issue records the completed protocol work; it does not track downstream VS Code integration.