fix: address Kong/Cobra CLI parity review feedback
- Make --help short-circuit before validation by using Kong's built-in
help flag (drop NoDefaultHelp + post-parse Help check). Help now prints
and exits 0 even with mutex/enum/arg/file validation errors, matching Cobra.
- Drop --setup-timeout from kong train (Cobra train has no such flag).
- Remove push mutual-exclusivity Validate (Cobra push has no such check).
- Drop type:"existingfile" from --dockerfile so file checks defer to build
time, matching Cobra and letting --help short-circuit.
