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/ui/expui/github/pullRequestClosed_dark.svg

@@ -0,0 +1,11 @@
+<!-- Copyright 2000-2026 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license. -->
+<svg fill="none" version="1.1" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg">
+  <g stroke="#DB5C5C" stroke-width="1.11">
+    <circle transform="scale(1,-1)" cx="3.97" cy="-12.4" r="2.22"/>
+    <circle transform="scale(1,-1)" cx="3.91" cy="-3.58" r="2.22"/>
+    <path d="m8.07 3.62h1.8c1.23 0 2.22 0.995 2.22 2.22v4.32"/>
+    <path d="m3.97 9.64v-3.73" stroke-linecap="round" stroke-linejoin="round"/>
+    <circle transform="scale(1,-1)" cx="12.1" cy="-12.4" r="2.22"/>
+    <path d="m10.1 5.87-2.22-2.22 2.22-2.22" stroke-linecap="round"/>
+  </g>
+</svg>