feat: add Description section for specific command help

[remorses]

When running --help on a specific command, shows a Description section with the full command description.

Changes:

• Show full description in Description section when running command --help
• Only show first line in commands listing to keep main --help compact

Example:

$ myapp some-command --help

Usage:
  $ myapp some-command

Description:
  Full multi-line description here
  with markdown formatting preserved

Options:
  --id <id>  Node ID

[Copilot]

Pull request overview

Adds richer, command-specific help output by introducing a dedicated Description section for subcommands, while keeping the main/global help output compact.

Changes:

• Add a “Description” section (full, multi-line) for non-global, non-default command --help output.
• Truncate command descriptions to the first line in the global/default “Commands” listing.

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[Copilot]

New behavior adds a dedicated "Description" section for subcommand help, but there are no tests asserting this output. Since help output is user-facing and already has coverage (e.g., examples/help.js in src/test/index.test.ts), please add a test (snapshot or string assertions) that verifies some-command --help includes the Description section and preserves multi-line descriptions.

[Copilot]

The commands listing now truncates descriptions to the first line. Please add/adjust tests to cover this formatting change (e.g., ensure a multi-line command description only shows the first line in the global/default --help output), so future refactors don't regress the compact help behavior.