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.

internal/auth/policy.go

@@ -93,6 +93,14 @@ func Requirement(m ipc.Method) Authority {
 		return AuthWrite
 	case ipc.MethodWriteFact:
 		return AuthWrite
+	case ipc.MethodSetTaskStatus:
+		// Resolving a task is NOT additive, which is what separates it from
+		// capture. Capture at AuthRead can only put a line on a list he reads
+		// himself; SetTaskStatus at AuthRead would let any enrolled module —
+		// mavpoll, mavsttd — mark every open task done and clear the list out
+		// from under him. Same reasoning as WriteFact: a module gets to add to
+		// its own corner, not to erase his.
+		return AuthWrite
 	case ipc.MethodAssertStepUp:
 		return AuthRead
 	case ipc.MethodLatestFact,
@@ -109,10 +117,11 @@ func Requirement(m ipc.Method) Authority {
 		// module write, not an allowlist mutation and not a new standing reason
 		// for Maven to speak — nothing in the tick loop reads tasks. It stays
 		// at AuthRead, the same rung as CreateReminder, which is the closest
-		// existing analogue.
+		// existing analogue. SetTaskStatus is NOT here: see the AuthWrite case
+		// above, because resolving is the one task move that destroys
+		// something.
 		ipc.MethodCaptureTask,
 		ipc.MethodListTasks,
-		ipc.MethodSetTaskStatus,
 		// Mail ingestion (Vikunja #246). AuthRead because of what the method can
 		// produce: candidate tasks and nothing else. It cannot write a fact, set a
 		// reminder, or touch the tool allowlist, so a compromised mail reader can