Remove label from GNode, move to node-label plugin

[Yonava]

Summary

• Removes label from the core GNode type — labels are a product-level concern, not a graph primitive
• Introduces @magic/graph-plugins package with a createNodeLabel plugin that manages label assignment, storage, and theme wiring
• Wires createNodeLabel into all product graphs via useGraph.ts
• Updates all theme callbacks across products to destructure { id } instead of receiving a full GNode
• Cleans up useAdjacencyList — removes labelAdjacencyList and fullNodeAdjacencyList which depended on GNode.label
• Removes newNodeLabelGetter from CoreGraphSettings and the old labels.ts composable
• Consolidates per-package .gitignore files into root

Known issues / follow-ups

• Bulk node additions (e.g. addElements) can assign duplicate labels due to a slice(0, -1) assumption in the label generator — tracked in [graph/plugins/node-label] bulk node additions assign duplicate labels #700
• treeArrayToGraph.ts still passes label to addNode with a @ts-expect-error migration comment; binary trees need a proper migration off label-based node ids
• Socket.io code in packages/server/src/sockets.ts is commented out (pre-existing, unrelated to this PR)