fix(folders): preserve parent folder in JSON output

[justkelvin]

Summary

Preserve ClickUp's documented parent_folder field in the JSON output of:

cup folders <space-id> --json

This allows consumers and AI agents to distinguish subfolders from top-level folders and reconstruct the folder hierarchy.

Problem

ClickUp's Get Folders API includes parent_folder for subfolders.

The API response reached the client intact, but the local Folder type did not expose the field and the folders command rebuilt each result using only:

• id
• name
• lists

As a result, cup folders --json flattened folders and subfolders without preserving their parent relationship.

Changes

• Add parent_folder?: string to the API folder type.
• Add parent_folder?: string to the folders command result type.
• Preserve the field only when ClickUp supplies it.
• Keep the existing flat-array JSON structure.
• Leave TTY and Markdown output unchanged.
• Add regression coverage for both top-level folders and subfolders.
• Document the JSON behavior in the README, command reference, and agent skill.

Example

[
  {
    "id": "100100",
    "name": "Acme Operations",
    "lists": []
  },
  {
    "id": "100110",
    "name": "Acme Launches",
    "parent_folder": "100100",
    "lists": []
  }
]

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[justkelvin]

Fixes #135