#20761: SDK-generated manifest attempts forbidden defaultValue mutation on system actor fields

After upgrading twenty-sdk to v2.6.0, app sync started failing with multiple FIELD_MUTATION_NOT_ALLOWED errors. Downgrading back to v2.5.0 did not resolve it.

The failing fields are system ACTOR fields automatically added to custom objects:

• createdBy
• updatedBy

Across several custom objects.

The generated manifest includes defaultValue for these system fields, e.g.:

{
  "name": "''",
  "source": "'MANUAL'"
}

But the server rejects updates to system fields unless only universalSettings or isActive are changed:

FIELD_MUTATION_NOT_ALLOWED: System fields only allow updating: universalSettings, isActive.
Forbidden properties: defaultValue

Read-only metadata inspection shows the remote fields already have a different defaultValue shape, so sync attempts to mutate it and fails. Since these fields are SDK-generated system fields, the SDK/installer should avoid including defaultValue in update payloads for system fields, or the diffing logic should ignore immutable system-field properties.

Workaround: strip defaultValue from generated system ACTOR fields before running install/sync.