Implement the issue state-transition mutations:
- issue edit: --title, --body/--body-file, --add-label/--remove-label,
--add-assignee/--remove-assignee, --milestone. Label mutations use
Gitea's dedicated additive/removal endpoints (names added directly,
removals resolved to IDs, unapplied-label 404 as silent success);
assignees use fetch-then-patch (ADR 0007); title/body/milestone and the
recomputed assignee list travel in one PATCH. Outputs edited: {number,
status: ok}.
- issue close: PATCHes state closed, optional --comment posted after with
its failure surfaced; already-closed short-circuits with Already closed.
- issue reopen: PATCHes state open; already-open short-circuits with
Already open.
Extract a shared getIssue helper used by view/edit/close/reopen.
