feat(beliefs): represent unknown explicitly; absence of evidence is not falsity

A belief with no live evidence on either side now derives the status
'unknown' instead of sitting tentatively at its prior. Withdrawing every
piece of evidence leaves the proposition unknown, never rejected, and
introspection answers 'I do not know whether X' rather than quoting the
prior. Hypotheses keep their own status because they are proposals by
construction. Migration 0019 widens the status constraint; the docs define
every epistemic number and how absence of evidence is handled.

internal/cognition/associate.go

@@ -1,8 +1,8 @@
 package cognition
 
 import (
-	"encoding/json"
 	"context"
+	"encoding/json"
 	"sort"
 	"strings"
 	"time"