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>

lib/builtin/decisions/src/extension.dart

@@ -20,6 +20,10 @@ class DecisionsExtension extends ClideExtension {
 
   @override
   Future<void> activate(ClideExtensionContext ctx) async {
+    // Ensure the retained nav exists so it records selections + emits
+    // loads whether or not the reader is mounted (T-196). This handler
+    // only reveals the tab; the nav owns load + history.
+    ctx.readerNav.navFor(id, dataKey: 'id');
     _sub = ctx.messages.subscribe(publisher: id, channel: 'selection').listen((msg) {
       if (msg.data['id'] is! String) return;
       ctx.arrangement.setVisible(Slots.contextPanel, true);