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/uk.ts

@@ -998,6 +998,8 @@ export const dict: Record<I18nKey, string> = {
   "gitView.pr.field.draft": "Чернетка",
   "gitView.pr.field.title": "Назва",
   "gitView.pr.githubNotConnected": "GitHub не підключено",
+  "gitView.pr.history.merged": "PR #{number} злито в {base}.",
+  "gitView.pr.history.closed": "PR #{number} закрито.",
   "gitView.pr.loadingDescription": "Завантаження опису...",
   "gitView.pr.mergeMethod.merge": "Створити коміт злиття",
   "gitView.pr.mergeMethod.rebase": "Перебазувати та злити",