Deep merge two configuration objects

Defaults, then workspace settings, then per-user overrides. With Object.assign a user who sets one nested option erases every other option in that branch, because the shallow copy replaces the whole sub-object.

Implement deepMerge(base, patch). Return a new object containing every key from base, with the keys from patch applied on top. Where both sides hold a plain object for the same key, merge those recursively. Where they do not, the value from patch replaces the one from base outright — so an array in patch replaces an array in base rather than concatenating with it, and a primitive in patch replaces an object in base. A key whose value in patch is undefined is skipped entirely and the value from base survives. A value of null is a real value and does overwrite. Neither input is modified.

deepMerge({ a: { x: 1, y: 2 } }, { a: { y: 9, z: 3 } });
// { a: { x: 1, y: 9, z: 3 } } — nested objects merge, not replace

deepMerge({ tags: [1, 2, 3] }, { tags: [9] });
// { tags: [9] } — arrays replace, they do not concatenate

deepMerge({ a: 1 }, { a: null });       // { a: null }  — null overwrites
deepMerge({ a: 1 }, { a: undefined });  // { a: 1 }     — undefined is skipped