Add preset describe, schema, and example commands

[mintyPT]

Problem

Users currently need to read the README or command help to understand a preset. There is no focused way to inspect one preset, its required options, defaults, media capabilities, or example usage.

Proposal

Add preset introspection commands:

clickclick preset describe gradient
clickclick preset schema gradient --json
clickclick preset example gradient

These should expose the same source of truth used by the CLI where possible.

Acceptance criteria

• preset describe <name> prints required options, optional options, defaults, and a short description.
• preset schema <name> --json emits machine-readable option metadata.
• preset example <name> prints an example CLI command and library usage.
• Invalid preset names return a stable error code and suggestions when possible.
• Tests cover at least one legacy preset and one media-backed preset.