feat(beliefs): temporal scope and validity; state changes supersede rather than contradict

Beliefs gain a temporal scope (persistent, transient or unknown, classified
deterministically from the proposition), an observation time and, for
transient states, a validity interval. When a newer belief conflicts with
an older transient one observed at least a minute earlier, the older belief
is closed (valid_until set, status superseded, a supersedes link, a version
and an event saying 'state changed') with its score untouched: 'the door
was open at 10:00' stays true of 10:00 when it is closed at 10:05.
Simultaneous conflicting reports and persistent propositions contradict as
before. Introspection lists superseded states with their validity.
Migration 0020 is additive.

migrations/0020_temporal_beliefs.down.sql

@@ -0,0 +1,6 @@
+DROP INDEX IF EXISTS beliefs_mind_subject_validity_idx;
+ALTER TABLE beliefs
+  DROP COLUMN IF EXISTS valid_until,
+  DROP COLUMN IF EXISTS valid_from,
+  DROP COLUMN IF EXISTS observed_at,
+  DROP COLUMN IF EXISTS temporal_scope;