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/ja.ts

@@ -994,6 +994,8 @@ export const dict: Record<I18nKey, string> = {
   'gitView.pr.field.draft': '下書き',
   'gitView.pr.field.title': 'タイトル',
   'gitView.pr.githubNotConnected': 'GitHubが接続されていません',
+  'gitView.pr.history.merged': 'PR #{number} は {base} にマージされました。',
+  'gitView.pr.history.closed': 'PR #{number} はクローズされました。',
   'gitView.pr.loadingDescription': '説明を読み込み中...',
   'gitView.pr.mergeMethod.merge': 'マージコミットを作成',
   'gitView.pr.mergeMethod.rebase': 'リベースしてマージ',