`System` enumerations are invisible to `show` / `describe` / `search`, so their values can only be found by guessing until CE1613

[MendixMau]

Summary

describe entity happily prints attributes typed against System enumerations, but the
enumerations themselves cannot be inspected through any mxcli command — so there is no way to
learn their valid values before the native build rejects a guess.

Environment

• mxcli: v0.22.0-15-g7b42100d (main HEAD, 2026-09-15)
• Mendix: 11.13.0, blank mxcli new scaffold
• Reproducible: yes, on any scaffold

Steps to reproduce

$ mxcli -p app.mpr describe entity System.WorkflowActivityRecord
  ...
  ActivityType: Enumeration(System.WorkflowActivityType),
  State:        Enumeration(System.WorkflowActivityExecutionState),
  ...

$ mxcli -p app.mpr describe enumeration System.WorkflowActivityType
Error: enumeration not found: System.WorkflowActivityType

$ mxcli -p app.mpr show enumerations          # 8 listed, none from System
$ mxcli -p app.mpr search "WorkflowActivityType"
No matches found.

A reasonable guess is then only caught by the native build:

[error] [CE1613] "The selected enumeration value
  'System.WorkflowActivityExecutionState.Finished' no longer exists."

Impact

Any MDL that branches on a System enumeration — workflow activity state, user-task completion
type, and anything else the platform models as an enum — has to be written by guess-and-build.
This blocked writing a diagnostic microflow over GET WORKFLOW ACTIVITY RECORDS entirely.

Same class as #1071 (check --references cannot resolve enumerations in an
attribute declaration), from the other direction: mxcli can read the model but will not tell you
what it read.

Suggested fix

Include System-module enumerations in show enumerations and describe enumeration — read-only
is fine, nobody needs to write them — and let search index them. Failing that, have the
MDL-side rule behind CE1613 list the values that do exist.