dialogue: a restart expires the parked question (V-385)

The decision, not a behaviour change: ClarifyStore stays in memory, and she
does not announce the loss either.

The TTL and the attempt count measure a pause in one conversation. A restart
is a gap of unknown length, so a restored question is either dead already or
lying about its age, and the request behind it is one he has likely given up
on. Announcing it would mean storing a marker that outlives the thing it
describes, to say one sentence in the rare window where he speaks within 90s
of a restart. His next words route fresh, which is right either way.

Written down in docs/design.md, pinned at both ends by a comment, and held by
a test that builds a second handler over the same store.

internal/dialogue/clarify.go

@@ -57,6 +57,14 @@ func (q *PendingQuestion) CanAsk() bool {
 
 // ClarifyStore holds the parked questions. Same shape and locking as
 // SessionStore: keyed by dialogue id, expired entries dropped on read.
+//
+// Memory only, deliberately, unlike SessionStore — a restart expires every
+// parked question and she does not announce that it happened (Vikunja #385,
+// written down in docs/design.md). The 90s TTL and the attempt count measure a
+// pause in one conversation, and a restart is a gap of unknown length, so a
+// restored question would either be dead already or lying about its age. His
+// next words route fresh, which is the right answer with or without a notice.
+// Do not give this store a persister without re-arguing that.
 type ClarifyStore struct {
 	mu         sync.RWMutex
 	questions  map[string]*PendingQuestion