Request: Expose CancelBackgroundTaskAsync via Copilot SDK RPC interface

[PureWeen]

Background

When a session runs a background subagent, the CLI tracks it internally via backgroundAgentRegistry (a Dbe instance). The registry has a cancel(agentId) method that fires the agent's AbortController, marks it cancelled, and triggers the change callback. However, this method is not exposed over the JSON-RPC interface that PolyPilot uses to communicate with the headless CLI.

The Problem

If a background agent never emits SubagentCompletedEvent or SubagentFailedEvent, PolyPilot's SessionIdleEvent keeps reporting backgroundTasks: { agents: 1 }. The IDLE-DEFER guard (PR #399) correctly blocks CompleteResponse while background tasks are active — but with a zombie subagent, it defers indefinitely until the watchdog's Case B stale checks fire (~6–30 minutes depending on events.jsonl activity).

The only recovery path today is a full session abort, which discards all accumulated response content.

What Already Exists in the CLI

// backgroundAgentRegistry (Dbe class) — internal to CLI process
cancel(agentId) {
    // fires AbortController, marks status = "cancelled", calls onChangeCallback
}

// Exposed on the session context
async cancelBackgroundTask(id) {
    return id.startsWith("agent-")
        ? this.backgroundAgentRegistry.cancel(id)
        : this.detachedShellRegistry.kill(id);
}

The LLM also has a list_agents tool (returns agentId, status, startedAt) and a read_agent tool, but no cancel_agent tool and no RPC method for callers outside the CLI process.

Request

Add a CancelBackgroundTaskAsync(string agentId) method to the Copilot SDK's CopilotSession (or equivalent RPC surface), backed by the existing cancelBackgroundTask implementation in the CLI.

This would allow PolyPilot to:

1. Detect a zombie subagent (started but no completion event after N minutes)
2. Call CancelBackgroundTaskAsync(agentId) to force the CLI to mark it cancelled and fire onChangeCallback
3. Receive a subsequent SessionIdleEvent with backgroundTasks: { agents: 0 } and complete normally — no abort required

Related

• IDLE-DEFER guard: CopilotService.Events.cs — HasActiveBackgroundTasks()
• SubagentStartedEvent / SubagentCompletedEvent / SubagentFailedEvent — already tracked in PolyPilot for UI feedback
• backgroundAgentRegistry.cancel() — CLI source, already implemented