Include parent and child metadata in issue JSON output

[dubscode]

Summary

Please include hierarchy metadata in normalized issue JSON returned by linear issues list and linear issues get.

Requested fields

At minimum:

{
  "parentId": "...",
  "parentIdentifier": "BB-418",
  "childrenCount": 7
}

Nice-to-have:

{
  "parentTitle": "evalite setup",
  "hasChildren": true
}

Why this matters

When working with Linear sub-issues, the current normalized issue output does not expose whether an issue is a parent or child. That makes it hard to safely build scripts that update only top-level issues, only children, or all children of a specific parent.

This pairs well with #3, but is useful independently: even without new filter flags, JSON consumers can make safer decisions if hierarchy fields are present.

Expected behavior

• linear issues get BB-418 --json should make it clear whether the issue has children.
• linear issues list --json should make it clear whether each row has a parent.
• Existing fields should remain backward-compatible.