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/vscode/CHANGELOG.md

@@ -1,7 +1,6 @@
 ## [Unreleased]
 
 - Chat: new chats no longer start against a deleted last worktree directory; they fall back to the active project instead of saving the first message and never starting.
-- Git: the pull request panel now follows the current open PR for the branch instead of keeping a merged or closed one after reload or a later open PR (thanks to @makeittech).
 
 ## [1.18.4] - 2026-08-14