fix: add precedes graph edge from classification to execution

Every run call creates a classification entity, but it was never
connected to the execution via a graph edge — only via a DB column
(executions.classification_id). The ontology requires:

    classification —precedes→ execution

Without this edge, all 53 classification entities had zero
relationships, making them invisible to get_relations and
blast-radius analysis.

Adds an idempotent INSERT into relationships after the existing
classification_id update, matching the same pattern used for
targets edges.

internal/mcp/server.go

@@ -921,6 +921,13 @@ func classifyAndGate(ctx context.Context, pool *db.Pool, agentID, targetID uuid.
 		classID, actionCol, riskClass, classRoute, classReason, correlationID)
 	// Link classification to execution.
 	pool.Exec(ctx, `UPDATE executions SET classification_id = $2 WHERE entity_id = $1`, id, classID)
+	// Graph edge: classification —precedes→ execution (required by ontology).
+	pool.Exec(ctx, `INSERT INTO relationships (source_id, target_id, type, attributes, valid_from)
+		SELECT $1, $2, 'precedes', '{"by":"nomos"}'::jsonb, now()
+		WHERE NOT EXISTS (
+			SELECT 1 FROM relationships
+			WHERE source_id = $1 AND target_id = $2 AND type = 'precedes' AND valid_to IS NULL)`,
+		classID, id)
 
 	// Audit: record the execution creation with session_id for traceability.
 	// Every run call, whether auto-run or queued-for-approval, gets an audit