fix: add involves edge from task to agent:nomos at creation

createTaskEntity creates the task entity but never adds any graph
edges. The involves edges are only added by the run handler, so
sessions that call set_goal but never make a run call leave orphan
task entities with zero relationships.

Adds an idempotent involves edge from the new task to agent:nomos
at creation time, matching the same pattern used for run's involves
edges in server.go.

cmd/nomos/store.go

@@ -179,6 +179,15 @@ func (s *store) createTaskEntity(ctx context.Context, sessionID, title string) s
 		`UPDATE agent_sessions SET entity_id = $1 WHERE id = $2`, entityID, sessionID); err != nil {
 		slog.Warn("nomos: could not link task entity", "session", sessionID, "error", err)
 	}
+	// Graph edge: task —involves→ agent:nomos (gives every task at least one
+	// edge from creation, even if no run calls are ever made).
+	s.pool.Exec(ctx, `INSERT INTO relationships (source_id, target_id, type, attributes, valid_from)
+		SELECT $1, id, 'involves', '{"by":"nomos"}'::jsonb, now()
+		FROM entities WHERE slug = 'agent:nomos'
+		AND NOT EXISTS (
+			SELECT 1 FROM relationships r
+			WHERE r.source_id = $1 AND r.target_id = entities.id AND r.type = 'involves' AND r.valid_to IS NULL)`,
+		entityID)
 	return entityID.String()
 }