Print help when required subcommands are omitted

[t-kalinowski]

Summary

• Make command switches required by default, so omitting a subcommand prints scoped help for the current command level.
• Add #| required: false as the explicit opt-out for apps or nested commands that intentionally allow running without a command.
• Update the bundled todo example, README guidance, and NEWS to describe the required-by-default command pattern.
• Add regression coverage with snapshot tests that capture the app source, invocation, and output in YAML form, including the opt-out path.

Closes #21.

Internal changes

• Record command-switch metadata from annotations during app parsing so runtime can tell whether a missing command should print help.
• Call the existing help renderer from argument processing when a required command is omitted, without adding a new public API surface.
• Update test fixtures that intentionally allow no command to opt out with #| required: false.

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