Route MPGv2 detach through the Machines API

[tripledoublev]

Summary

• route fly mpg detach through the public Machines API
• fall back to the legacy MPGv2 API only for classified 404 responses
• preserve existing detach output and secret-warning behavior
• update the fly-go dependency to released v0.9.13
• add coverage for public success, classified-404 fallback, fallback errors, and authoritative public errors

Testing

• go test ./internal/command/mpg/v2 -run Detach -count=1
• go test ./internal/command/mpg/... -count=1
• go mod tidy -diff
• go mod verify
• go vet ./...
• gofmt -l .
• git diff --check
• real disposable personal-org validation with a local build of this branch

[tomr-a5]

LGTM!

[tripledoublev]

thanks for the review @tomr-a5 !