* feat(app): add toolbar and loading states to the GitHub PR panel
Adds a toolbar under the PR title with a refresh button (refreshes git
and GitHub status + timeline in one tap), an always-visible View Pull
Request button, and a Merge control with a method dropdown that is
disabled-with-reason when the PR can't be merged yet.
The merge control reuses the existing git-actions policy verbatim via a
new buildPrPanelMergeActions selector fed into GitActionsSplitButton, so
no merge readiness/squash/auto-merge logic is duplicated.
Also replaces the empty-state flash with skeletons: a full-panel
skeleton while PR status loads and an activity skeleton while the
timeline loads, with empty states shown only when loaded-and-empty and
an error state with retry.
* refactor(app): tidy PR panel skeleton (i18n, shared pulse, index)
Addresses review feedback on the PR panel work:
- Localize the skeleton's "Checks" label via t() instead of hardcoding.
- Hoist the shared SkeletonPulse + useSkeletonPulse into activity-skeleton
so pane-skeleton reuses them instead of duplicating the pulse driver.
- Export PullRequestPaneError/Skeleton from the module index and import
them through it, matching the module's existing public surface.
* test(app): remove mock-based PR pane test
* fix(app): simplify PR panel toolbar
* fix(app): report PR panel retry failures
