feat: add pr edit, close, and reopen (task 0011)

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`.

src/assignees.ts

@@ -0,0 +1,44 @@
+/** The subset of a Gitea `User` this module reads: just the login, if present. */
+interface AssigneeLike {
+  login?: string;
+}
+
+/**
+ * The logins of an entity's current assignees, dropping any without one. The
+ * shared read side of fetch-then-patch: `issue edit` and `pr edit` both take the
+ * `assignees` off a freshly fetched entity and feed the result to
+ * {@link mergeAssignees}.
+ */
+export function assigneeLogins(assignees: AssigneeLike[] | undefined): string[] {
+  return (assignees ?? []).flatMap((assignee) => (assignee.login ? [assignee.login] : []));
+}
+
+/**
+ * The full assignee login list to PATCH under fetch-then-patch semantics
+ * (ADR 0007): the entity's current assignees with the requested additions
+ * appended and removals dropped. Matching is case-insensitive and the result is
+ * de-duplicated, order-preserving, so a login already assigned never lands in the
+ * list twice. Shared by `issue edit` and `pr edit`, whose PATCH bodies both
+ * replace the whole assignee list rather than adding or removing individual
+ * entries.
+ */
+export function mergeAssignees(current: string[], add: string[], remove: string[]): string[] {
+  const removeSet = new Set(remove.map((login) => login.toLowerCase()));
+  const seen = new Set<string>();
+  const result: string[] = [];
+  const push = (login: string): void => {
+    const key = login.toLowerCase();
+    if (removeSet.has(key) || seen.has(key)) {
+      return;
+    }
+    seen.add(key);
+    result.push(login);
+  };
+  for (const login of current) {
+    push(login);
+  }
+  for (const login of add) {
+    push(login);
+  }
+  return result;
+}