preflight should flag an old-form simplycubed caller, not skip it #140

Found by the post-rebrand review (#137). checkMentionAgreement (cmd/aixgo/main.go:481) recognizes a caller only by the new needle aixgo-dev/code/.github/workflows/aixgo.yml@. A repo mid-migration that still carries a caller pinned to simplycubed/code/.github/workflows/simplycubed.yml@v0.3.0 is invisible to preflight: it finds zero callers, returns nil, and reports success — while comment commands never fire. That is exactly the silent failure this check exists to turn into a loud error.

preflight should also match the old-form uses: string and fail with a message that names the file and says to re-run aixgo init --workflow and delete the old caller.