fix: hide kong predict/train/weights/debug and drop dead os.Exit
- Add hidden:"" tags to predict, train, weights, and debug commands to
match Cobra's Hidden: true (run stays visible).
- Remove unreachable os.Exit(1) after parser.FatalIfErrorf(err), which
already exits the process.
- Update root help test to assert hidden commands are hidden in the
Kong model rather than absent as substrings.
