Show merged PR status when no open PR exists

- extend `GitStatusResult` with nullable `mergedPr` metadata
- in server git status, query merged PR only when no open PR is found
- in sidebar, display a `Merged` status pill for threads on branches with merged PRs
- update server/web/contracts tests for the new status shape
46347e2bd2Julius Marminge committed on 2/15/2026, 10:04:48 PMparent7fc53da
8 files changedLine totals unavailable