The docs for accessing context from a node are here
In summary, to access flow context you use:
let flowContext = this.context().flow
let value = flowContext.get("value")
One thing you must be aware of when using context in a custom node is that the user might have configured a context store that requires asynchronous only access. The Change node deals with this under the covers for you, but in your node, you'd need to use the async functions as described here.