Add a comment to a Jira Cloud issue in Atlassian Document Format (ADF), including an @mention and restricted visibility

Documented steps

1. Understand the format split: /rest/api/2/ takes a plain string comment body (wiki markup); /rest/api/3/ requires an ADF document object. Sending a string to v3 fails with 'Comment body is not valid!'.
2. Build the minimal ADF document: {"type":"doc","version":1,"content":[{"type":"paragraph","content":[{"type":"text","text":"Deployment completed."}]}]}
3. POST it: POST /rest/api/3/issue/{issueIdOrKey}/comment with Content-Type: application/json and body {"body": <the ADF doc above>}.
4. To @mention a user, first resolve their accountId (GET /rest/api/3/user/search?query=name), then insert a mention node into a paragraph's content array: {"type":"mention","attrs":{"id":"<accountId>","text":"@Display Name"}}.
5. Mix text and mention nodes inside the same paragraph content array to get 'Hi @Alice, build failed.' — each node is a separate array entry.
6. To restrict who can see the comment, add a sibling 'visibility' object: {"type":"role","value":"Administrators","identifier":"Administrators"} for a project role, or type 'group' with the group ID as identifier.
7. Read comments back: GET /rest/api/3/issue/{issueIdOrKey}/comment?startAt=0&maxResults=100. orderBy accepts only 'created'.
8. If you need HTML rather than ADF JSON when reading, add expand=renderedBody — the 'body' field itself always stays ADF.