Add unit tests for merge-utils module

[nikolasdehor]

Problem

The merge-utils module (.aios-core/core/config/merge-utils.js) has no unit test coverage. This module implements the deep merge strategy from ADR-PRO-002 with +append arrays, null deletion, and layered config merging.

Solution

Add 24 unit tests covering all merge strategies and edge cases.

Test Details

Area	Tests	Key Scenarios
isPlainObject	6	Objects, null, arrays, primitives, class instances
deepMerge	12	Scalars, nested, arrays, +append, null delete, immutability
mergeAll	6	Multiple layers, zero/single, null/non-object skip
Total	24

[nikolasdehor]

Covered by consolidated PR #424