Leave a delivery cut off by a stop for the next start (closes #527)

Stopping the engine cancels the context every http and slack request is
sent with. An attempt cut off that way was recorded as failed, which for
a target with no retries, or on a last retry, ended the delivery failed
and nothing sent it again.

The shared retry core now records nothing for a failed attempt when the
engine's context is cancelled: no result, no circuit breaker outcome, no
status change. The delivery stays pending or retrying and startup
recovery sends it again. A target's own timeout is still a failed
attempt. The README's Graceful shutdown bullet, Shutdown section and the
at-least-once note say so.

Model: opus-5-5

internal/delivery/target_slack.go

@@ -82,7 +82,7 @@ func (t *slackTarget) Deliver(
 	}
 
 	t.deliver(
-		webhookDB, d, task, sched,
+		ctx, webhookDB, d, task, sched,
 		d.Target.MaxRetries, attempt,
 	)
 }