bug: cancelled expert tasks leave no trace in taskboard

[cameronsjo]

Found during dogfooding

The docker expert was cancelled (user denied the tool call during a batch), but there's no cancelled status in the taskboard — the task just disappeared. The concierge had no way to know it needed to retry.

Expected

Cancelled tasks should appear in the taskboard with status: cancelled so the concierge can detect them via pool_check_status or pool_collect and decide whether to retry.

Root cause

When the user denies a tool call in Claude Code, the MCP tool call simply doesn't execute. The concierge never calls pool_dispatch, so no message reaches the postoffice, and no task is registered. From the daemon's perspective, nothing happened.

Fix options

1. Concierge-side tracking — the concierge tracks dispatched IDs locally and detects missing ones when collecting
2. Two-phase dispatch — register the task in taskboard first (pending), then write to postoffice. If the tool call is denied, the pending task times out or is detected as orphaned
3. Accept it — document that cancelled dispatches are invisible. The concierge should re-dispatch if pool_collect returns not_found

Option 3 is probably right for now — pool_collect already returns not_found status.