Link two Jira Cloud issues and create subtasks or epic children with the parent field

Documented steps

1. Discover valid link types first — they are configured per site, not globally fixed: GET /rest/api/3/issueLinkType returns {"issueLinkTypes":[{"id":"1010","name":"Blocks","inward":"is blocked by","outward":"blocks"}, ...]}.
2. Create the link: POST /rest/api/3/issueLink with body {"type":{"name":"Blocks"},"inwardIssue":{"key":"ABC-2"},"outwardIssue":{"key":"ABC-1"}}. Direction matters: outwardIssue performs the action described by 'outward' on inwardIssue.
3. Optionally attach a comment to the link event by adding a 'comment' object with an ADF 'body'.
4. Expect an empty response body. If you need the link's ID, re-fetch the issue: GET /rest/api/3/issue/ABC-1?fields=issuelinks.
5. Create a subtask: POST /rest/api/3/issue with fields {"project":{"key":"ABC"},"issuetype":{"id":"<subtask type id>"},"parent":{"key":"ABC-1"},"summary":"..."}.
6. Find the subtask issue type id from GET /rest/api/3/issue/createmeta/{projectIdOrKey}/issuetypes — do not assume 'Sub-task' has a fixed id.
7. For epic children, use the same 'parent' field ({"parent":{"key":"ABC-100"}}) rather than the legacy Epic Link custom field. 'parent' is the unified forward-compatible path across team-managed and company-managed projects.
8. Re-parent an existing issue with PUT /rest/api/3/issue/{key} and fields.parent.