feat(github): pull request tool window, timeline and review flow (#477)

Replaces the Pull Requests tool window with a full review workflow on top of gh's authentication.

- List with search, quick filters and paging; a tab per pull request and a New Pull Request tab.
- Details: commit picker, changes tree with viewed state, CI and reviewer status, merge, close and reopen.
- Timeline editor: comments, review threads, reactions and suggested changes.
- Review in the diff viewer and, for the checked-out branch, in the editor (review mode).
- New generic vcs extension points: registerDiffViewerProvider, lineInlays, reviewGutter, branch widget extensions; new vcs_merge_base and gh_api commands.
- CSP: img-src allows https://avatars.githubusercontent.com for avatars.

Fixed while testing against a real repository: remotes behind an SSH host alias, mergeability stuck on UNKNOWN, Apply Locally staying disabled after a checkout, relative links navigating the app's page, and review inlays wider than the pane.

Co-authored-by: imacross <[email protected]>

src/features/github/ui/Placeholder.tsx

@@ -0,0 +1,15 @@
+/** The marked placeholder of a part of the port that is not built yet (foundation stubs). */
+import type { ReactNode } from "react";
+import { T } from "@/ui";
+import styles from "./ui.module.css";
+
+export function Placeholder({ title, children }: { title: string; children?: ReactNode }) {
+  return (
+    <div className={styles.placeholder} data-placeholder={title}>
+      <div className={styles.placeholderTitle}>
+        <T>{title}</T>
+      </div>
+      {children}
+    </div>
+  );
+}