Add the remaining simple issue mutations. `issue delete` hard-deletes via
the DELETE endpoint and is deliberately not idempotent — a nonexistent
issue yields ISSUE_NOT_FOUND rather than reporting success. `issue pin`
and `issue unpin` read the current pin state (Gitea's pin_order field) and
short-circuit to an idempotent no-op with an Already pinned/unpinned
message when there is nothing to do.
