phase 4: fix execution FK violation — create entity row before insert

- phase3.go: RequestExecution now calls InsertEntity before InsertExecution
  (executions.entity_id references entities.id via FK constraint).
- mcp/server.go: request_execution MCP tool same fix — inserts entities row
  with slug 'exec:<target>:<id8>' before executions insert.
- docker-compose.yml: fix seed OIKOS_SEEDS_DIR from /app/seeds to /seeds
  (distroless image COPY destination).

internal/httpapi/phase3.go

@@ -504,6 +504,18 @@ func (s *Server) RequestExecution(ctx context.Context, req gen.RequestExecutionR
 	defer tx.Rollback(ctx)
 
 	q := sqlcgen.New(tx)
+
+	execSlug := "exec:" + id.String()[:8]
+	if _, err := q.InsertEntity(ctx, sqlcgen.InsertEntityParams{
+		ID:         id,
+		Slug:       execSlug,
+		Type:       "execution",
+		Name:       req.Body.Action + " on " + req.Body.Target,
+		Attributes: []byte("{}"),
+	}); err != nil {
+		return nil, err
+	}
+
 	if err := q.InsertExecution(ctx, sqlcgen.InsertExecutionParams{
 		EntityID:       id,
 		TargetEntityID: &targetID,