- Prompt for secret values when name provided without value
- Display asterisks per character for input length verification
- Handle Ctrl+C, backspace, and non-printable character filtering
- Maintain backwards compatibility with NAME=VALUE inline syntax
- Skip SUPABASE_ prefixed names before prompting

- Check read byte count before accessing buffer to fix G602 gosec warnings
- Explicitly discard term.Restore error to fix errcheck warning

Bytes >= 128 were silently dropped, corrupting UTF-8 secrets.