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).

docker-compose.yml

@@ -42,7 +42,7 @@ services:
         condition: service_completed_successfully
     environment:
       OIKOS_DATABASE_URL: postgres://oikos:${OIKOS_DB_PASSWORD:-oikos_dev}@postgres:5432/oikos?sslmode=disable
-      OIKOS_SEEDS_DIR: /app/seeds
+      OIKOS_SEEDS_DIR: /seeds
     command: ["seed"]
     restart: "no"