fix(sandbox): move update banner to stderr; add functional test suite

- sandbox update: move "Sandbox updated" banner to eprintln! (stderr)
  so -o json stdout is clean — same fix as sandbox new
- scripts/test_commands.sh: new comprehensive functional test script
  covering all 128 command/subcommand/flag/output-format combinations,
  with real API calls, resource lifecycle, and cleanup

Author: eddietejeda