feat(graph): association is not causation

Adds co_occurs_with, temporally_precedes, possibly_caused_by and
asserted_by to the relation vocabulary with Causal and Hypothetical
predicates. Entities mentioned together now co-occur rather than being
'related'; association hypotheses record possibly_caused_by and their
basis (temporal association only); caused_by is reserved for explicit
operator statements and nothing automatic writes it. Documented in
docs/graph.md and checked end to end.

internal/cognition/entities.go

@@ -45,10 +45,12 @@ func (h Handlers) linkEntities(ctx context.Context, c *brain.Cycle, names []stri
 			add("", n)
 		}
 	}
-	// The speaker discussed each entity; mentioned entities relate to each other.
+	// The speaker discussed each entity; entities mentioned together merely
+	// co-occur. Co-occurrence is association, never a causal or structural
+	// claim; stronger relations need an explicit statement or an operator.
 	for i := range ents {
 		for j := i + 1; j < len(ents); j++ {
-			rel := concepts.RelatedTo
+			rel := concepts.CoOccursWith
 			if ents[i].Kind == graph.Person && i == 0 && actor != "" {
 				rel = concepts.DiscussedWith
 			}