Stop all in-flight Tally work in one conversation

Cancels every queued and running turn in the conversation and closes its active Bookkeeping Startup goal so background reconciliation cannot resume the work. Explicit program abandon (Cancel from the firm-wide In progress list, or Clear on an ordinary chat before force-new): running workers lose the turn lease immediately, stop at their next tool boundary, and write nothing back. Active Back work / QuickBooks import program claims are released so the process leaves In progress. It is not a pause state — the next message in any conversation starts a fresh turn normally. Cancelling a conversation with no active work is a safe no-op. Clear beside an active import uses /clear-chat instead so the artifact survives.