Request: Lint for mentors syntax as part of a dry-run

[joestringer]

This code describes that some linter will fail if mentors are not part of members:

team-manager/pkg/config/config.go

Line 267 in f887243

// Note 1: Mentors _must_ be in the member list. Lint will warn if they are not

The relevant code appears to be here, but it actually only checks if the person is a member of the organization:

team-manager/pkg/team/utils.go

Line 34 in f887243

fmt.Printf("[ERROR] mentor %q from team %s, not found in the list of team members in the organization\n", member, teamName)

This code should:

1. Also check that the mentor is a member of the specific team, in addition to being a member of the org
2. Return an error up the stack so that the error can prevent silent success
3. Eventually print the error to stderr, not stdout
4. Report the failure to the user when dry-run flag is provided
5. Be available as part of a linter without running a 'push' command

I found this out because I made this "wrong syntax" in the following PR:

cilium/community#235

The dry-run for applying these settings did not complain:

https://github.com/cilium/team-management/actions/runs/15142421235/job/42569714896#step:7:1