chore: add ADO expression check to trigger.pipeline fields or document why it's omitted

[jamesadevine]

Problem

validate_front_matter_identity checks name/description for both newlines AND ADO expressions (${{, $(, $[), but the new trigger.pipeline field validation (#184) only checks for newlines.

While resources.pipelines fields (source:, project:, branch entries) are structural config values that ADO likely doesn't evaluate as expressions at runtime, the inconsistency with the established pattern could confuse future maintainers.

Suggested Fix

Either:

1. Add the ADO expression check to trigger.pipeline fields too (defense in depth), or
2. Add a code comment explaining why it's intentionally omitted for these fields

Context

Found during review of #184.