fix decision first-click + editor reveal via retained reader nav

Two reveal-on-open bugs:

Decisions opened only on the second click (T-196): the detail view
subscribed in didChangeDependencies, which runs after the tab is
revealed, so the broadcast 'selection' that triggered the reveal was
already gone. Hoist the back/forward history out of per-view State into
a retained per-reader ReaderNav (kernel ChangeNotifier in a
ReaderNavRegistry, D-81). The nav records selections, emits 'load' (the
single channel readers display from), and survives mount/unmount — the
reader grabs nav.current on mount, so the first selection lands. Both
the markdown and decisions readers move to this model; the per-view
ReaderHistoryMixin and the markdown post-frame forward hack are gone.

The editor pane never opened (T-197): EditorExtension contributed a
workspace tab but nothing activated it on editor.open. Add an activate()
that reveals the tab on editor.opened / editor.active-changed; the
view's hydrate() pulls the active buffer on mount.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/builtin/markdown/markdown_viewer_test.dart

@@ -50,9 +50,10 @@ Future<void> pumpView(WidgetTester tester, KernelFixture f) async {
   await pumpAsync(tester);
 }
 
-/// Trigger a file load via the 'load' channel (mirrors the extension bridge).
+/// Open a file through the retained nav (the history source); its 'load'
+/// emit makes the mounted viewer display it (T-196).
 Future<void> loadFile(WidgetTester tester, KernelFixture f, String path) async {
-  f.services.messages.publish('builtin.markdown', 'load', {'path': path});
+  f.services.readerNav.navFor('builtin.markdown', dataKey: 'path').open(path);
   await pumpAsync(tester);
 }