Allow users to set `background_task.syncPollTimeoutMs` in config to override
the default 10-minute sync subagent timeout. Affects sync task, sync continuation,
and unstable agent task paths. Minimum value: 60000ms (1 minute).
Co-authored-by: Wine Fox <fox@ling.plus>
Tasks with no progress.lastUpdate were silently skipped in
checkAndInterruptStaleTasks, causing them to hang forever when the model
hangs before its first tool call. Now falls back to checking startedAt
against a configurable messageStalenessTimeoutMs (default: 10 minutes).
Closes#1769