Improve GitHub panel toolbar and loading states (#1664)

* 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

packages/app/src/i18n/resources.test.ts

@@ -243,6 +243,7 @@ describe("translation resources", () => {
     expect(en.workspace.git.actions.commit.label).toBe("Commit");
     expect(en.workspace.git.diff.binaryFile).toBe("Binary file");
     expect(en.workspace.git.pr.sections.checks).toBe("Checks");
+    expect(en.workspace.git.pr.actions.viewPullRequest).toBe("View");
     expect(en.review.comment.placeholder).toBe("Leave a comment");
   });