fix(github): keep merged PRs as branch history instead of hiding them

Branch status resolves an open PR across the whole fork network first, so a
merged fork PR can never hide an open upstream PR for the same head. Only when
no target has an open PR does the branch's newest closed/merged PR come back,
as history.

The panel shows that history as a compact note and offers creating the next PR
below it, instead of either sticking on a terminal PR or going blank after a
merge. Terminal associations stay persisted for reload continuity but are never
treated as authority: they revalidate on the discovery cadence and on focus.

History is looked up only for the branch's own remote and name, and remembered
per repo+branch, so the extra lookup cannot exhaust the route's resolve budget.
The checks summary and merge-permission lookup are skipped for a closed or
merged PR, where neither is actionable.

packages/ui/src/lib/i18n/messages/ko.ts

@@ -998,6 +998,8 @@ export const dict: Record<I18nKey, string> = {
   'gitView.pr.field.draft': '드래프트',
   'gitView.pr.field.title': '제목',
   'gitView.pr.githubNotConnected': 'GitHub에 연결되지 않음',
+  'gitView.pr.history.merged': 'PR #{number}이(가) {base}에 병합되었습니다.',
+  'gitView.pr.history.closed': 'PR #{number}이(가) 닫혔습니다.',
   'gitView.pr.loadingDescription': '설명 로드 중…',
   'gitView.pr.mergeMethod.merge': '병합 커밋 생성',
   'gitView.pr.mergeMethod.rebase': '리베이스 후 병합',