Allow discovery of teams configuration files in subfolders

* Perhaps use a configurable glob style syntax for discovery?
* https://docs.microsoft.com/en-us/dotnet/core/extensions/file-globbing#get-all-matching-files
* Must flatten full list to validate no duplicates
* If duplicate detected, fail operation
* Failure of an operation would be reported either by the Action or the GitHub App as a commit/PR status. Do not embed reporting logic in Core

Why allow this? To leverage CODEOWNERS as a means to delegate trust for team creation

Start by locating the current team-configuration discovery entry point and tracing how configuration files are loaded and validated. Define the expected glob behavior, flattening and duplicate-detection rules, then verify that failure reporting remains outside Core as described in the issue.

Written by the indexing model from the issue text.