Add the PR-side state mutations mirroring the issue-side slice:
- `pr edit` applies title/body/base/milestone and the recomputed assignee
list in one PATCH, with additive label endpoints and (per the ADR 0007
amendment) the dedicated requested-reviewers POST/DELETE endpoints for
`--add-reviewer`/`--remove-reviewer`.
- `pr close --comment` posts the comment after the PATCH and surfaces a
comment-post failure; an already-closed or merged PR is an `already: true`
no-op reporting the actual state.
- `pr reopen` is an `already: true` no-op when already open.
Extract the fetch-then-patch assignee merge into a shared `src/assignees.ts`
(`mergeAssignees` + `assigneeLogins`), now used by both `issue edit` and
`pr edit`.
