tasks: key derived captures by external id and record who resolved

A task extracted from mail deduped on the live-norm index only, so once he
finished it the row left the live set and the next poll of the same immutable
message re-extracted it as a fresh candidate. mavmaild is a read-only reader
and marks nothing read, so that repeats forever. Derived rows now carry an
ext_id built from the message uid and the extracted span, unique across every
status, while voice keeps live-only norm dedupe because saying an errand again
is the recurrence signal. A derived source can no longer capture straight to
open, and saying a task out loud that Maven had only proposed promotes the
candidate instead of answering that it is already in the list.

SetTaskStatus was classified AuthRead. Resolving a task is not additive, it
erases work off his list, so it is a write, and the row now records the caller
that moved it. ListTasks was unbounded. The list-query matcher claimed any
utterance with "что мне делать", including "с чем мне помочь", and the urgency
stripper matched inside words.

Found in review of #60.

cmd/mavend/actions_task.go

@@ -42,6 +42,12 @@ func (h *reactiveHandler) captureTaskFromNote(ctx context.Context, dec router.De
 		log.Printf("voice: capture task: %v", err)
 		return "не получилось записать задачу.", true
 	}
+	if resp.Promoted {
+		// It was a candidate Maven derived from something she read, and he has
+		// now said it himself. Saying "уже в списке" here would be answering a
+		// confirmation with a shrug.
+		return "поняла, беру в работу: " + cap.Text, true
+	}
 	if !resp.Created {
 		return "это уже в списке.", true
 	}