- Add _deepMergeConfig() with correct null-override semantics
- loadConfig() reads root config.json when GSD_WORKSTREAM is set, then
deep-merges with workstream config (workstream wins on conflict)
- Workstream without config.json falls back to root config entirely
- Migrations and disk writes operate on fileData (on-disk content) only,
never on the merged result, to prevent workstream pollution
- Fixes null-override bug from PR #2717: explicit null in workstream now
correctly overrides root value instead of falling back to root
- Tests: inherit root model_overrides, workstream override, nested
workflow.* deep merge, explicit null override, missing workstream config
Closes#2714
1cb4bebcf5