Scope subcommand usage help

- Avoid overwhelming `UsageError` output for commands with subcommands.
- Keep root command help to subcommand summaries and root flags.
- Reuse parser metadata so matched subcommands show valid flags only.
- Group generated manpage options with their matching subcommands.
- Scope generated shell completions to root and subcommand flags.
- Use operation-specific flag text in help, manpages and completions.